Display apparatus

ABSTRACT

Disclosed herein is a display apparatus has a pixel array section including: pixel circuits which are each provided with a driving transistor and an electro-optical device and are laid out to form a matrix; and a draw wire provided in each of the pixel circuits to serve as a wire connecting the driving transistor to a power-supply providing line, wherein the resistance of the draw wire is relatively large in the pixel circuit close to a source applying a power-supply voltage to the power-supply providing line.

CROSS REFERENCES TO RELATED APPLICATIONS

This is a Continuation application of U.S. patent application Ser. No.12/385,189, filed Apr. 1, 2009, which claims priority from JapanesePatent Application JP 2008-106456 filed with the Japanese Patent Officeon Apr. 16, 2008 the entire contents of which being incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display apparatus employing pixelcircuits each having an electro-optical device. The pixel circuit isalso referred to merely as a pixel whereas the electro-optical device isalso referred to as a display device or a light emitting device. To putit in more detail, the present invention relates to a display apparatusin which each of the pixel circuits employs an electro-optical device toserve as an electrically driven display device emitting light with aluminance varying in accordance with the magnitude of a driving signaldriving the device and employs an active element for controlling thedriving signal so that the display apparatus can be driven to display animage in pixel units.

2. Description of the Related Art

There is a display apparatus employing pixel circuits each having anelectro-optical device serving as a display device which emits lightwith a luminance varying in accordance with the magnitude of a drivingsignal which is a voltage applied to the device or a current flowingthrough the device. For example, a liquid crystal display device is atypical electro-optical device which emits light with a luminancevarying in accordance with the magnitude of a voltage applied to thedevice. On the other hand, an organic EL (Electro Luminescence) deviceand an OLED (Organic Light Emitting Diode) are each a typicalelectro-optical device which emits light with a luminance varying inaccordance with the magnitude of a current flowing through the device. Adisplay apparatus employing pixel circuits each including an organic ELdevice is referred to as an organic EL display apparatus which is adisplay apparatus of the so-called light-self-emission type. A displayapparatus of the light-self-emission type is a display apparatusemploying pixel circuits each including an electro-optical device of thelight-self-emission type.

The organic EL device has an organic thin film which is also referred toas an organic layer sandwiched by a lower electrode and an upperelectrode. The organic layer is a laminated layer composed of an organichole transport layer and an organic light emitting layer. The organic ELdevice is an electro-optical device utilizing a phenomenon in whichlight is emitted when an electric field is applied to the organic thinfilm. Thus, by controlling a current flowing through the organic ELdevice, it is possible to obtain gradations of emitted colors.

Since the organic EL device can be driven by a relatively low voltagesuch as a voltage not exceeding 10 V, the organic EL device has a lowpower consumption. In addition, since the organic EL device is a lightself-emitting device which emits light by itself, the organic EL displayapparatus employing pixel circuits each having an organic EL device doesnot require an auxiliary illumination member such as a backlight whichis needed in a liquid-crystal display apparatus. It is thus easy toreduce the weight and thickness of the organic EL display apparatusemploying pixel circuits which each have an organic EL device. On top ofthat, since the response speed of the organic EL device is extremelyhigh, a residual image of a moving-image display is not generated. Atypical response time of the organic EL device has a value of the orderof several microseconds. Due to the merits offered by the organic ELdevice as described above, a planar light-self-emission displayapparatus, which employs pixel circuits each having an organic EL deviceto serve as an electro-optical device, is developed intensively andextensively in recent years.

By the way, a display apparatus employing pixel circuits each having anelectro-optical device can adopt a simple (passive) matrix method or anactive matrix method as a driving method. Representative examples of thedisplay apparatus employing pixel circuits each having anelectro-optical device are a liquid-crystal display apparatus employingpixel circuits each having a liquid-crystal display device and anorganic EL display apparatus employing pixel circuits each having anorganic EL device. However, even though a display apparatus adopting thesimple matrix method has a simple structure, such a display apparatusraises a problem that it is difficult to design the display apparatusinto a display apparatus having a large size or a high-definitiondisplay apparatus.

In order to solve the problem described above, the active matrix methodis developed intensively and extensively in recent years. In accordancewith the active matrix method, a pixel signal supplied to a lightemitting device employed in a pixel circuit is controlled by making useof an active device, which is employed in the same pixel circuit as thelight emitting device, as a switching device. A typical example of theactive device is an insulated-gate FET (Field Effect Transistor) whichis generally a TFT (Thin Film Transistor).

In order to drive an electro-optical device of such a pixel circuit toemit light, an input image signal is supplied to a signal holdingcapacitor employed in the pixel circuit to serve as a capacitor forholding the input image signal. The input image signal is supplied tothe signal holding capacitor employed in the pixel circuit through avideo-signal line. In the pixel circuit, the input image signal issupplied from the video-signal line to the gate electrode of a drivingtransistor for driving the electro-optical device as well as a signalholding capacitor connected to the gate electrode. The input imagesignal is supplied from the video-signal line to the gate electrode ofthe driving transistor as well as the signal holding capacitor by way ofa switching transistor which is also referred to as a samplingtransistor. In this way, a driving signal according to the voltage ofthe input image signal held in the signal holding capacitor is thensupplied to the electro-optical device. It is to be noted that, in thefollowing description, the gate electrode of the driving transistor isalso referred to as a control input terminal of the driving transistorwhereas the signal holding capacitor is also referred to as a pixelcapacitor.

In a liquid-crystal display apparatus employing pixel circuits eachhaving a liquid-crystal display device to serve as an electro-opticaldevice, the liquid-crystal display device is a device of avoltage-driven-emission type. Thus, the liquid-crystal display device isdriven by a voltage signal according to the input image signal held inthe signal holding capacitor. In an organic EL display apparatusemploying pixel circuits each having a current-driven-emission devicesuch as an organic EL device to serve as an electro-optical device, onthe other hand, the voltage signal according to the input image signalheld in the signal holding capacitor is converted by the drivingtransistor into a current signal and the organic EL device is driven bythe current signal.

Represented by an organic EL device, the electro-optical device of thecurrent-driven-emission type is a device emitting light with a luminancewhich varies as the current driving the device varies. In order to drivethe electro-optical device to emit light with a stable luminance, it isimportant to supply a stable driving current to the electro-opticaldevice. Methods for supplying a driving current to an organic EL deviceare classified typically into two large categories, i.e., aconstant-current driving method and a constant-voltage driving method.Since each of the constant-current driving method and theconstant-voltage driving method is a method based on a known technology,however, the references made available to the public as referencesdisclosing these methods are not given in this patent specification.

Since the voltage-current characteristic of the organic EL device has alarge gradient, execution of the constant-voltage driving method givesrise to large current variations due to even small variations in voltageand/or small variations in device characteristic. The large variationsin current in turn bring about large variations in luminance. For thisreason, it is generally necessary to adopt the constant-current drivingmethod which makes use of the driving transistor in a saturated region.Of course, even in the case of the constant-current driving method, ifvariations in current exist, such variations will result in variationsin luminance. If the variations in current are small, however, thevariations in luminance are also small as well.

Conversely, even if the constant-current driving method is adopted, inorder to make the luminance of light emitted by the electro-opticaldevice stable, it is important to sustain the driving signal, which isbeing held in the signal holding capacitor as a signal according to theinput image signal, at a fixed magnitude. For example, in order to makethe luminance of light emitted by the organic EL device stable, it isimportant to sustain the driving current, which is flowing through thedriving transistor as a current according to the input image signal, ata fixed magnitude.

By the way, variations in manufacturing process inevitably give rise tothreshold-voltage and mobility variations of an active device fordriving the electro-optical device. In addition, the characteristic ofthe electro-optical device such as the organic EL device also changeswith the lapse of time. These threshold-voltage and mobility variationsof the active device for driving the electro-optical device and thesechanges of the characteristic of the electro-optical device undesirablyhave an effect on the luminance of light emitted by the electro-opticaldevice.

For the reasons described above, in order to control the luminance oflight emitted by the electro-optical device to a value uniformthroughout the entire display screen of the display apparatus, there hasbeen studied a variety of mechanisms for compensating each pixel circuitfor the threshold-voltage and mobility variations of the active deviceemployed in the pixel circuit to serve as a device for driving theelectro-optical device and for the changes of the characteristic of theelectro-optical device of the pixel circuit.

For example, a mechanism described in Japanese Patent Laid-Open No.2007-310311 (hereinafter referred to as Patent Document 1) has threeproposed functions, i.e., a threshold-voltage compensation function, amobility compensation function and a bootstrap function, for a pixelcircuit employing an organic EL device. The threshold-voltagecompensation function is a function for sustaining the driving currentgenerated by the driving transistor at a constant magnitude, given aconstant input image signal, even if the threshold voltage of thedriving transistor varies from transistor to transistor and changes withthe lapse of time. The mobility compensation function is a function forsustaining the driving current generated by the driving transistor at aconstant magnitude, given a constant input image signal, even if themobility of the driving transistor varies from transistor to transistorand changes with the lapse of time. The bootstrap function is a functionfor sustaining the driving current generated by the driving transistorat a constant magnitude, given a constant input image signal, even ifthe current-voltage characteristic of the organic EL device changes withthe lapse of time.

SUMMARY OF THE INVENTION

In the mechanism described in Patent Document 1, however, the drainelectrode of the driving transistor is connected to a power-supplyproviding line to which a pulse power-supply voltage is applied in avertical scanning operation. The drain electrode of the drivingtransistor is also referred to as a power-supply providing terminal ofthe driving transistor or a power-supply providing terminal of the pixelcircuit whereas the power-supply providing line is a typical verticalscanning line. Since it is necessary to provide thecurrent-driven-emission light emitting device such as the organic ELdevice with a current, which is sufficiently large for driving the lightemitting device to emit light, a relatively large current flows from apower-supply generation source through the power-supply providing lineto the light emitting device by way of the driving transistor. Thus, apower-supply voltage generated by a power-supply generation source suchas a driving scanning section and applied to the power-supply providingline decreases by a drop which is gradually increasing with the distancebetween a location (at which the power-supply voltage is observed) onthe power-supply providing line and the power-supply generation source.As a result, an electric potential appearing on the power-supplyproviding terminal of the driving transistor is lower than thepower-supply voltage appearing on the power-supply generation source bya difference, and the longer the distance from the power-supplygeneration source to the driving transistor, the larger the difference.

In such a case, the longer the distance from the power-supply generationsource to a driving transistor, the smaller the voltage appearingbetween the drain and source electrodes of the driving transistor. Thatis to say, the voltage appearing between the drain and source electrodesof a driving transistor decreases gradually from pixel to pixel as thedistance between the pixel circuit and the power-supply generationsource increases. Thus, the pixel circuit including the transistor isaffected by the so-called early effect. For this reason, even if themethod described in Patent Document 1 as a method for eliminatingvariations in characteristics is adopted, the driving current generatedby a driving transistor gradually decreases as the distance between thedriving transistor and the power-supply generation source increases.Thus, the luminance of light emitted by the light emitting device alsoundesirably decreases gradually as well. If the luminance of lightemitted by the light emitting device decreases gradually, luminanceirregularities such as shadings and crosstalk are generated unavoidably.In the case of a color display apparatus, the luminance irregularitiesappear on the screen as color irregularities.

Addressing the problem described above, inventors of the presentinvention have innovated a mechanism capable of preventing displayirregularities such as luminance and color irregularities from beinggenerated due to drops of voltages appearing on power-supply providingterminals of driving transistors.

An embodiment implementing a display apparatus provided by the presentinvention has a pixel array section including: pixel circuits which areeach provided with a driving transistor and an electro-optical deviceand are laid out to form a matrix; and a draw wire provided in each ofthe pixel circuits to serve as a wire connecting the driving transistorto a power-supply providing line, wherein the resistance of the drawwire is relatively large in the pixel circuit close to a source applyinga power-supply voltage to the power-supply providing line.

For example, under a condition to drive the electro-optical devicesemployed in the pixel circuits to emit light with a uniform luminance,at least one of the length and width of any one of the draw wires areset so that the electric potentials appearing on the power-supplyproviding terminals of the driving transistors become uniform for allthe driving circuits.

In a display apparatus having a pixel array section including pixelcircuits each employing a current-driven-emission electro-opticaldevice, the length and width of any one of the draw wires each providedin one of the pixel circuits arranged in the longitudinal direction ofthe power-supply providing line are adjusted in such a way that, under acondition to drive the light emitting devices employed in the pixelcircuits to emit light with a uniform luminance, at least one of thelength and width of any one of the draw wires are set so that theelectric potentials appearing on the power-supply providing terminals ofthe driving transistors become uniform for all the driving circuits.

In accordance with the embodiment of the present invention, the drawwires are created to form a layout in the pixel circuits by adjusting atleast one of the resistance of any one of the draw wires at such varyingvalues that, under a condition to drive the light emitting devicesemployed in the pixel circuits to emit light with a uniform luminance,at least one of the length and width of any one of the draw wires areset so that the electric potentials appearing on the power-supplyproviding terminals of the driving transistors become uniform for allthe driving circuits. Thus, it is possible to prevent luminanceirregularities such as shadings and crosstalk from being generated dueto voltage drops along power-supply providing lines. As a result, adisplay apparatus having a high image quality can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the embodiments of the present inventionwill become clear from the following description of the preferredembodiments given with reference to the accompanying diagrams, in which:

FIG. 1 is a block diagram roughly showing a typical configuration of anactive-matrix organic EL display apparatus according to an embodiment ofthe present invention;

FIG. 2 is a diagram showing a first typical comparison configuration fora pixel circuit according to this embodiment;

FIG. 3A is an explanatory diagram to be referred to in describing anoperating point of the driving transistor and the organic EL device;

FIG. 3B is an explanatory diagram to be referred to in describingeffects, which the changes of characteristics of the driving transistorand the organic EL device have on a driving current;

FIG. 4 is a diagram showing a second typical comparison configurationfor the pixel circuit according to this embodiment;

FIG. 5 is a diagram showing the pixel circuit according to thisembodiment;

FIG. 6 is a timing diagram showing a plurality of explanatory timingcharts to be referred to in description of basic driving timings of theembodiment shown in the diagram of FIG. 5 as an embodiment implementingthe pixel circuit;

FIG. 7A is an explanatory circuit diagram to be referred to indescription of an equivalent circuit and operating state of the pixelcircuit in a period (A) shown in the timing charts of FIG. 6;

FIG. 7B is an explanatory circuit diagram to be referred to indescription of an equivalent circuit and operating state of the pixelcircuit in a period (B) shown in the timing charts of FIG. 6;

FIG. 7C is an explanatory circuit diagram to be referred to indescription of an equivalent circuit and operating state of the pixelcircuit in a period (C) shown in the timing charts of FIG. 6;

FIG. 7D is an explanatory circuit diagram to be referred to indescription of an equivalent circuit and operating state of the pixelcircuit in a period (D) shown in the timing charts of FIG. 6;

FIG. 7E is an explanatory circuit diagram to be referred to indescription of an equivalent circuit and operating state of the pixelcircuit in a period (E) shown in the timing charts of FIG. 6;

FIG. 7F is an explanatory circuit diagram to be referred to indescription of an equivalent circuit and operating state of the pixelcircuit in a period (F) shown in the timing charts of FIG. 6;

FIG. 7G is an explanatory circuit diagram to be referred to indescription of an equivalent circuit and operating state of the pixelcircuit in a period (G) shown in the timing charts of FIG. 6;

FIG. 8 is an explanatory diagram to be referred to in description of aproblem caused by the wire resistance of a power-supply providing lineserving as a power-supply line to appear as a problem raised in anoperation to display an image at an all white display time;

FIG. 9A is an explanatory circuit diagram to be referred to indescription of effects of the wire resistance of the power-supplyproviding line;

FIG. 9B is an explanatory diagram showing the Vds-Ids characteristic ofa driving transistor;

FIG. 9C is an explanatory diagram showing a phenomenon which is observedwhen a window pattern is displayed;

FIG. 10 is an explanatory conceptual diagram to be referred to indescription of a mechanism provided in accordance with a variety ofembodiments to serve as a mechanism for repressing displayirregularities caused by voltage drops at a plurality of points alongthe power-supply providing line;

FIG. 11A is a circuit diagram showing a pixel circuit according to theembodiments and a typical comparison example for the pixel circuit;

FIG. 11B is a diagram showing a pixel-component layout of a typicalcomparison example for the pixel circuit;

FIG. 12 is an explanatory diagram serving as a model diagram to bereferred to in description of a typical layout according to the firstembodiment implementing the basic principle explained earlier byreferring to the diagram of FIG. 10 by changing the length of a seconddraw wire in each pixel circuit;

FIG. 13A is an explanatory diagram serving as a model diagram to bereferred to in description of a typical layout according to the secondembodiment implementing the basic principle explained earlier byreferring to the diagram of FIG. 10 by changing the width of a draw wirein each pixel circuit; and

FIG. 13B is an explanatory diagram serving as a model diagram to bereferred to in description of a typical layout according to the thirdembodiment implementing the basic principle explained earlier byreferring to the diagram of FIG. 10 by changing the length and width ofa second draw wire in each pixel circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention are described in detail byreferring to diagrams as follows.

<Outline of the Entire Display Apparatus>

FIG. 1 is a block diagram roughly showing a typical configuration of anactive-matrix organic EL display apparatus 1 according to an embodimentof the present invention. The embodiment to be explained below is apresent-invention implementation in which, in each pixel circuitincluded in the typical configuration shown in the block diagram of thefigure, as an example, an organic EL device of thecurrent-driven-emission type is used as a display device whereas apoly-silicon TFT (Thin Film Transistor) is used as the active device fordriving the organic EL device. The embodiment implements anactive-matrix organic EL display apparatus built up by creating organicEL devices on a semiconductor substrate on which the poly-silicon TFTshave been formed. As described earlier, in the following description,the display device is also referred to as an electro-optical device or alight emitting device whereas the active-matrix organic EL displayapparatus is also referred to as an organic EL display apparatus or justa display apparatus. Typically, the TFT is an FET (Field EffectTransistor).

The display apparatus 1 is used as a display section in various kinds ofelectronic equipment, which employs a recording medium such as asemiconductor memory, an MD (MiniDisc, Registered Trademark of SonyCorporation) or a cassette tape, in all fields. Examples of suchelectronic equipment are a portable music player, a digital camera, anotebook personal computer, a portable terminal such as a cellular phoneand a video camera. The display section is a section for displaying avideo signal supplied to the electronic apparatus or a video signalgenerated in the electronic apparatus as a standstill or moving picture(image).

It is to be noted that, in the following description explaining thewhole configuration of the display apparatus 1, an organic EL device ofa pixel circuit is used as a display device of the pixel circuit.However, the organic EL device is no more than a typical display device.That is to say, the display device is by no means limited to the organicEL device. For example, any electro-optical device can generally be usedas the display device as far as the electro-optical device is acurrent-driven-emission electro-optical device. In addition, the displaydevice used in this embodiment can also be used in all other embodimentsdescribed later or, in particular, in embodiments each implementing acountermeasure against a drop of from a voltage generated by thepower-supply generation source to the voltage appearing on thepower-supply providing terminal of the driving transistor.

As shown in the block diagram of FIG. 1, the display apparatus 1 employsa display panel section 100, a driving-signal generation section 200 anda video-signal processing section 220. The display panel section 100 hasa pixel array section 102 which serves as a main component. The pixelarray section 102 includes a plurality of pixel circuits P laid out toform a pixel matrix in an effective video area having a display aspectratio of X:Y. Also referred to as a pixel P, each of the pixel circuitsP has an organic EL device serving as a display device which is notshown in the block diagram of FIG. 1. The display aspect ratio of theeffective video area is defined as the ratio of the height X of the areato the width Y of the area. A typical value of the display aspect ratiois 9:16. Referred to as the so-called timing generator, thedriving-signal generation section 200 is a typical example of a panelcontrol section for generating a variety of panel signals forcontrolling and driving the display panel section 100. Thedriving-signal generation section 200 and the video-signal processingsection 220 are embedded in a single-chip IC (Integrated Circuit). Inthe case of this embodiment, the single-chip IC including thedriving-signal generation section 200 and the video-signal processingsection 220 is installed externally to the display panel section 100.

In the case of the configuration shown in the block diagram of FIG. 1,in the display panel section 100, the pixel array section 102 describedabove is put on a substrate 101. A vertical-direction driving section103 and a horizontal-direction driving section 106 are mounted on thesubstrate 101. The vertical-direction driving section 103 is a sectionfor scanning the pixel circuits P in the vertical direction whereas thehorizontal-direction driving section 106 is a section for scanning thepixel circuits P in the horizontal direction. The horizontal-directiondriving section 106 is also referred to as a horizontal selector or adata-line driving section. On top of that, a terminal section 108 usedfor connecting the display panel section 100 to the external componentssuch as the driving-signal generation section 200 and the video-signalprocessing section 220 is placed on an edge of a side of the displaypanel section 100. The terminal section 108 also referred to as a padsection. It is to be noted that, if required, an interface (IF) sectionserving as an interface between external circuits and thevertical-direction driving section 103 as well as between externalcircuits and the horizontal-direction driving section 106 may be mountedin some cases.

Typically, the vertical-direction driving section 103 employs a writescanning section 104 and a driving scanning section 105. The writescanning section 104 is also referred to as a write scanner WS. Alsoreferred to as a drive scanner DS, the driving scanning section 105functions as a power-supply scanner which has a power-supply providingpower. As an example, the pixel array section 102 is driven by thevertical-direction driving section 103 including the write scanningsection 104 and the driving scanning section 105 from one side of thepixel array section 102 or two sides separated away from each other inthe left-to-right direction of the block diagram of FIG. 1 as the twosides of the pixel array section 102. By the same token, the pixel arraysection 102 is driven by the horizontal-direction driving section 106from one side of the pixel array section 102 or two sides separated awayfrom each other in the top-down or bottom-up direction of the bockdiagram of FIG. 1 as the two sides of the pixel array section 102.

The vertical-direction driving section 103, which employs the writescanning section 104 and the driving scanning section 105, and thehorizontal-direction driving section 106 form a control section 109 forcontrolling a write operation to store the electric potential of asignal into the signal holding capacitor. The control section 109 alsocontrols a threshold-voltage compensation operation, a mobilitycompensation operation and a bootstrap operation which will be describedlater in detail. The control section 109 is thus a circuit for drivingthe pixel circuits P of the pixel array section 102.

As described above, in the configuration of this implementation, theperipheral driving circuits such as the vertical-direction drivingsection 103 and the horizontal-direction driving section 106 are mountedon the substrate 101 which is the same substrate as the pixel arraysection 102.

In the typical configuration shown in the block diagram of FIG. 1, thedriving-signal generation section 200 serving as a signal sourceexternal to the display apparatus 1 supplies a variety of pulse timingsignals to the display panel section 100 by way of the terminal section108. However, the driving-signal generation section 200 can also becreated as a semiconductor chip mounted on the display panel section100.

As described above, the driving-signal generation section 200 serving asa signal source external to the display apparatus 1 supplies a varietyof pulse timing signals to the display panel section 100 by way of theterminal section 108. In the same way, the video-signal processingsection 220 serving as a signal source external to the display apparatus1 supplies a video signal Vsig to the display panel section 100 by wayof the terminal section 108. In the case of a color display apparatus 1,the video signal Vsig includes video signals of three colors, i.e., ared video signal Vsig_R, a green video signal Vsig_G and a blue videosignal Vsig_B.

As an example, vertical write/drive pulse signals output from theterminal section 108 include necessary pulse signals such asvertical-direction shift start pulses SPDS and SPWS as well asvertical-direction scanning clock signals CKDS and CKWS. If required,the vertical write/drive pulse signals may include also verticalscanning clock signals xCKDS and xCKWS which have phases opposite to thephases of the vertical scanning clock signals CKDS and CKWSrespectively.

By the same token, horizontal-direction driving pulse signals outputfrom the terminal section 108 include necessary pulse signals such as ahorizontal-direction start pulse SPH and a horizontal-direction clocksignal CKH. If required, the horizontal-direction driving pulse signalsmay include also a horizontal-direction clock signal xCKH which has aphase opposite to the phase of the horizontal-direction clock signalCKH.

Terminals of the terminal section 108 are connected to thevertical-direction driving section 103 and the horizontal-directiondriving section 106 by signal lines 199. If necessary, the voltage levelof each pulse supplied to the terminal section 108 from thedriving-signal generation section 200 and the video-signal processingsection 220 is typically adjusted by a level shifting section embeddedin the terminal section 108 before being supplied to thevertical-direction driving section 103 and the horizontal-directiondriving section 106 by way of a buffer also embedded in the terminalsection 108. It is to be noted that the level shifting section and thebuffer themselves are not shown in the block diagram of FIG. 1.

The pixel array section 102 employs pixel circuits P arrangedtwo-dimensionally to form a pixel matrix. Each of the pixel circuits Pincludes an organic EL device serving as a display device and a pixeltransistor serving as a driving transistor for driving the organic ELdevice as will be described later in detail. The organic EL device andthe driving transistor themselves are not shown in the block diagram ofFIG. 1. For each pixel row of the pixel matrix, a pair of scanning lines104WS and 105DSL are provided. By the same token, for each pixel columnof the pixel matrix, a signal line 106HS is provided.

In the typical configuration of the pixel array section 102, thescanning lines 104WS and 105DSL are created as scanning lines used inthe vertical-direction scanning operation whereas the signal line 106HSis created as a video-signal line (or a data line) used in thehorizontal-direction scanning operation. At each intersection of avertical-direction scanning line pair, which includes thevertical-direction scanning lines 104WS and 105DSL, and the video-signalline 106HS used in the horizontal-direction scanning operation, anorganic EL device and a TFT serving as the driving transistor fordriving the organic EL device are created. The organic EL device and thedriving transistor are combined to form a pixel circuit P. As describedabove, the organic EL device and the driving transistor themselves arenot shown in the block diagram of FIG. 1.

To put concretely, for the pixel circuits P laid out to form the pixelmatrix, write scanning lines 104WS_1 to 104WS_n are provided. To put itin detail, the write scanning lines 104WS_1 to 104WS_n are providedrespectively for the n pixel rows of the pixel matrix. Each of the writescanning lines 104WS_1 to 104WS_n is a line for propagating a writedriving pulse WS generated by the write scanning section 104.

By the same token, for the pixel circuits P laid out to form the pixelmatrix, power-supply providing lines 105DSL_1 to 105DSL_n are provided.To put it in detail, the power-supply providing lines 105DSL_1 to105DSL_n are provided respectively for the n pixel rows of the pixelmatrix. Each of the power-supply providing lines 105DSL_1 to 105DSL_n isa line for propagating a power-supply driving pulse DSL generated by thedriving scanning section 105.

On the other hand, for the pixel circuits P laid out to form the pixelmatrix, video-signal lines 106HS_1 to 106HS_m are provided. To put it indetail, the video signal lines 106HS_1 to 106HS_m are providedrespectively for the m pixel columns of the pixel matrix. Each of thevideo signal lines 106HS_1 to 106HS_m is a line for propagating a videosignal Vsig generated by the horizontal-direction driving section 106.

Each of the write scanning section 104 and the driving scanning section105 is designed as a combination of logic gates forming latches andregisters. Each of the write scanning section 104 and the drivingscanning section 105 selects a matrix row of pixel circuits P employedin the pixel array section 102. That is to say, the write scanningsection 104 selects one of the write scanning lines 104WS_1 to 104WS_nsequentially on a line-after-line basis in accordance with a pulsesignal received from the driving-signal generation section 200 as apulse signal generated for a vertical-direction driving system.

By the same token, the driving scanning section 105 selects one of thepower-supply providing lines 105DSL_1 to 105DSL_n sequentially on aline-after-line basis in accordance with a pulse signal received fromthe driving-signal generation section 200 as a pulse signal generatedfor a vertical-direction driving system.

In the same way as the write scanning section 104 and the drivingscanning section 105, the horizontal-direction driving section 106 isdesigned as a combination of logic gates forming latches and registers.However, the horizontal-direction driving section 106 selects a matrixcolumn of pixel circuits P employed in the pixel array section 102. Thatis to say, the horizontal-direction driving section 106 selects one ofthe video signal lines 106HS_1 to 106HS_m sequentially on aline-after-line basis in accordance with a pulse signal received fromthe driving-signal generation section 200 as a pulse signal generatedfor a horizontal-direction driving system. The horizontal-directiondriving section 106 samples a predetermined electric potential of thevideo signal Vsig and stores the sampled electric potential into thesignal holding capacitor, which is employed in each of the pixelcircuits P provided on the selected matrix column associated with theselected video signal line 106HS, through the selected video signal line106HS.

The display apparatus 1 according to this embodiment can be drivensequentially on a line-after-line basis or a point-after-point basis. Toput it in detail, each of the write scanning section 104 and the drivingscanning section 105, which are employed in the vertical-directiondriving section 103, scans the pixel array section 102 sequentially on aline-after-line basis, that is, on a row-after-row basis. Insynchronization with the scanning operations carried out by the writescanning section 104 and the driving scanning section 105, thehorizontal-direction driving section 106 supplies image signals of onepixel row to the pixel array section 102 at the same time (in the caseof a line-after-line driving operation) or supplies an image signal foreach pixel circuit P to the pixel array section 102 (in the case of apoint-after-point driving operation).

It is to be noted that a display-apparatus product is by no meanslimited to the display apparatus 1 built up as a module of compoundcomponents including the display panel section 100, the driving-signalgeneration section 200 and the video-signal processing section 220 asshown in the block diagram of FIG. 1. For example, it is possible topresent a display apparatus including only the display panel section 100or only the pixel array section 102.

As a typical example, the display apparatus 1 includes a module of asealed configuration. As another typical example, the display apparatus1 is configured as a display module including only the display panelsection 100 created by attaching a transparent facing glass plate on thepixel array section 102. On the transparent facing glass plate, adisplay layer, a color filter, a protection film, a light shielding filmand other layers/films are created. In the case of this embodiment, thedisplay layer is an organic layer and electrodes provided on both sidesof the layer. In the case of the configuration of this display panelsection 100, in addition to the pixel array section 102, electricalconnection terminals are provided on an edge of the display panelsection 100. The electrical connection terminals are external connectionterminals for electrically connecting the pixel array section 102 to anexternal component such as an FPC (Flexible Print Circuit) having amounted circuit. The circuit mounted on the FPC has the same functionsas the vertical-direction driving section 103 and thehorizontal-direction driving section 106. As described earlier, thevertical-direction driving section 103 and the horizontal-directiondriving section 106 receive a variety of driving pulses and a videosignal Vsig from external sources and supply the driving pulses and thevideo signal Vsig to the pixel array section 102. With regard to theother components, the configuration of this display panel section 100 isbasically identical with the configuration shown in the block diagram ofFIG. 1.

As described earlier, in the configuration shown in the block diagram ofFIG. 1, the pixel array section 102 is driven by the vertical-directiondriving section 103 including the write scanning section 104 and thedriving scanning section 105 from only one side of the pixel arraysection 102. As an alternative, the pixel array section 102 can also bedriven by the vertical-direction driving section 103 from two sidesseparated away from each other in the left-to-right direction of thebock diagram of FIG. 1 as the two sides of the pixel array section 102.In this case, two vertical-direction driving sections 103 sandwich thepixel array section 102.

By the same token, in the configuration shown in the block diagram ofFIG. 1, the pixel array section 102 is driven by thehorizontal-direction driving section 106 from only one side of the pixelarray section 102. As an alternative, the pixel array section 102 canalso be driven by the horizontal-direction driving section 106 from twosides separated away from each other in the top-down or bottom-updirection of the bock diagram of FIG. 1 as the two sides of the pixelarray section 102. In this case, two horizontal-direction drivingsections 106 sandwich the pixel array section 102.

<First Typical Comparison Configuration of the Pixel Circuit>

FIG. 2 is a diagram showing a first typical comparison configuration forthe pixel circuit P according to this embodiment. It is to be noted thatthe diagram of FIG. 2 also shows the write scanning section 104, thevertical-direction driving section 103 and the horizontal-directiondriving section 106, which are located on peripheries of the pixel arraysection 102 placed on the substrate 101 of the display panel section100.

Each of transistors employed in the first typical comparisonconfiguration of the pixel circuit P is a MOS (Metal OxideSemiconductor) FET (Field Effect Transistor). One of the transistorsemployed in the first typical comparison configuration is a drivingtransistor 121. The gate electrode G of the driving transistor 121functions as a control input terminal. A specific one of the sourceelectrode S and drain electrode D of the driving transistor 121 servesas an input terminal whereas the other one of the source electrode S andthe drain electrode D is used as an output terminal. In particular, usedas a source for supplying a driving current Ids to an organic EL device127, the source electrode S of the driving transistor 121 functions asthe output terminal whereas the drain electrode D of the drivingtransistor 121 serves as the input terminal which is a power-supplyproviding terminal. The following description explains a typical pixelcircuit P having a configuration including two transistors. Theconfiguration including two transistors is also referred to as a2TR-driven configuration.

Since the organic EL device 127 is a current-driven-emission lightemitting device, the gradation of a generated color is obtained bycontrolling the magnitude of a driving current Ids flowing through theorganic EL device 127. The first typical comparison configurationincluding two transistors as shown in the diagram of FIG. 2 as theconfiguration of the pixel circuit P is a conceivably simplest circuit.In the first typical comparison configuration, the driving transistor121 is a p-channel FET whereas a sampling transistor 125 also referredto as a write transistor is an n-channel FET. In the followingdescription, a p-channel FET is referred to as a Pch-type FET whereas ann-channel FET is referred to as an Nch-type FET.

The source electrode S of the Pch-type driving transistor 121 isconnected to a power supply whereas the drain electrode D of thePch-type driving transistor 121 is connected to the anode electrode ofthe organic EL device 127. The cathode electrode of the organic ELdevice 127 is connected to a cathode wire Wcath which is normally aground wire GND. The gate electrode G of the driving transistor 121 isconnected to the horizontal-direction driving section 106 through avideo-signal line 106HS and the sampling transistor 125. A signalholding capacitor 120 is connected between the power supply and the gateelectrode G of the driving transistor 121.

The horizontal-direction driving section 106 supplies a video signalVsig to the gate electrode G of the driving transistor 121 through thevideo-signal line 106HS and the sampling transistor 125. By changing thevoltage of the video signal Vsig supplied to the gate electrode G of thedriving transistor 121, the magnitude of a driving current Ids flowingfrom the driving transistor 121 to the organic EL device 127 can becontrolled. As described above, the source electrode S of the Pch-typedriving transistor 121 is connected to the power supply. The drivingtransistor 121 is designed to operate in a saturated region in which thedriving current Ids generated by the driving transistor 121 has amagnitude independent of a voltage appearing between the drain andsource electrodes of the driving transistor 121.

Let reference notation Ids denote a current flowing between the drainand source electrodes of the driving transistor 121 operating in asaturated region, reference notation μ denote the mobility of thedriving transistor 121, reference notation W denote the channel width(or the gate width) of the driving transistor 121, reference notation Ldenote the channel length (or the gate length) of the driving transistor121, reference notation Cox denote the gate capacity (or the gateoxidation film capacity per unit area) of the driving transistor 121 andreference notation Vth denote the threshold voltage of the drivingtransistor 121. In this case, the driving current Ids generated by thedriving transistor 121 functioning as a constant-current generator isexpressed by Eq. (1) given below. It is to be noted that symbol “^” usedin the equation denotes the exponentiation operator. As is obvious fromEq. (1), in the saturated region, the driving transistor 121 functionsas a constant-current generator which generates a driving current Idscontrolled by a gate-source voltage Vgs appearing between the gate andsource electrodes of the driving transistor 121.

$\begin{matrix}{{Ids} = {\frac{1}{2}\mu\frac{W}{L}{{Cox}\left( {V_{gs} - V_{th}} \right)}^{\bigwedge}2}} & (1)\end{matrix}$<Change of Characteristic and the Effects of the Changes>

FIGS. 3A and 3B are explanatory diagrams referred to in describingchanges of characteristics of pixel-configuration devices (that is, thedriving transistor 121 and the organic EL device 127) and effects of thechanges. To be more specific, FIG. 3A is an explanatory diagram referredto in describing an operating point of the driving transistor 121 andthe organic EL device 127. On the other hand, FIG. 3B is an explanatorydiagram referred to in describing effects, which the changes ofcharacteristics of the driving transistor 121 and the organic EL device127 have on the driving current Ids.

<I-V Characteristic of the Light Emitting Device>

In general, the I-V characteristic of a current-driven-emission lightemitting device represented by an organic EL device deteriorates withthe lapse of time as shown in the diagram of FIG. 3B. The I-Vcharacteristic shown in the diagram of FIG. 3B is a relation between acurrent Iel flowing through the current-driven-emission light emittingdevice represented by an organic EL device and a voltage Vel applied tothe device. A curve shown as a solid line represents the Iel−Velcharacteristic exhibited by the organic EL device at an initial time. Onthe other hand, a curve shown as a dashed line represents the Iel−Velcharacteristic exhibited by the organic EL device after the lapse oftime.

For example, when a light-emission current Iel is flowing through theorganic EL device 127 serving as a typical light emitting device, theanode-cathode voltage Vel appearing between the anode and cathodeelectrodes of the organic EL device 127 is determined uniquely. Thedriving current Ids is a current flowing between the drain and sourceelectrodes of the driving transistor 121 in a light emission period. Thedriving current Ids flows to the anode electrode of the organic ELdevice 127 as the light-emission current Iel which determines theanode-cathode voltage Vel of the organic EL device 127 uniquely. Withthe lapse of time, however, the anode-cathode voltage Vel changes by adifference of (Vel2−Vel1) shown in the diagram of FIG. 3B even if thelight-emission current Iel is sustained at a constant magnitude.

Since the I-V characteristic of the organic EL device 127 changes withthe lapse of time as described above, the dashed-line curve shown in thediagram of FIG. 3A as the curve for the organic EL device 127 is shiftedto the left, also shifting the position of an operating point shown inthe diagram of FIG. 3A to the left and changing the voltage appearing onthe drain electrode D of the driving transistor 121 employed in the samefirst typical comparison configuration of the pixel circuit P shown inthe diagram of FIG. 2 as the organic EL device 127. Since the drivingtransistor 121 is operating in a saturated region, however, the drivingcurrent Ids generated by the driving transistor 121 has a magnitudeindependent of the voltage appearing on the drain electrode D of thedriving transistor 121. That is to say, the driving current Idsgenerated by the driving transistor 121 has a magnitude dependent on thegate-source voltage Vgs of the driving transistor 121. Since thegate-source voltage Vgs of the driving transistor 121 does not changewith the lapse of time, however, the driving current Ids having amagnitude, which does not change with the lapse of time, continues toflow to the organic EL device 127. Thus, the luminance of light emittedby the organic EL device 127 also does not change with the lapse oftime.

<Second Typical Comparison Configuration of the Pixel Circuit>

FIG. 4 is a diagram showing a second typical comparison configurationfor the pixel circuit P according to this embodiment. It is to be notedthat the diagram of FIG. 4 also shows the write scanning section of thevertical-direction driving section 103 and the horizontal-directiondriving section 106, which are located on peripheries of the pixel arraysection 102 placed on the substrate 101 of the display panel section100.

In the second typical comparison configuration of the pixel circuit P,an Nch-type FET is employed as the driving transistor 121 to serve as asubstitute for the Pch-type driving transistor 121 of the first typicalcomparison configuration of the pixel circuit P. Since the drivingtransistor 121 has the same type as the sampling transistor 125, theprocess of manufacturing the pixel circuit P becomes simpler. However,the second typical comparison configuration of the pixel circuit Praises a problem described as follows. In the case of the second typicalcomparison configuration of the pixel circuit P, the drain electrode Dof the driving transistor 121 is connected to the power supply whereasthe source electrode S of the driving transistor 121 is connected to theanode electrode of the organic EL device 127. With the lapse of time,the Iel−Vel characteristic of the organic EL device 127 deteriorates asdescribed above and the anode-cathode voltage Vel changes by adifference of (Vel2−Vel1) shown in the diagram of FIG. 3B even if thelight-emission current Iel remains unchanged. The change of the I-Vcharacteristic of the organic EL device 127 also changes the position ofan operating point shown in the diagram of FIG. 3A and the voltageappearing on the source electrode S of the driving transistor 121 evenif the voltage applied to the gate electrode G of the driving transistor121 remains the same. Thus, the gate-source voltage Vgs of the drivingtransistor 121 also inevitably changes. Since the magnitude of thedriving current Ids is determined by the gate-source voltage Vgs of thedriving transistor 121 as is obvious from Eq. (1) serving as acharacteristic equation, the magnitude of the driving current Ids alsounavoidably changes even if the voltage Vg applied to the gate electrodeG of the driving transistor 121 remains the same. Thus, thelight-emission current Iel flowing through the organic EL device 127also unavoidably changes, undesirably causing the luminance of lightemitted by the organic EL device 127 to change with the lapse of time.

As described above, in the pixel circuit P having the second typicalcomparison configuration, the electric potential appearing on the anodeelectrode of the organic EL device 127 serving as a typical lightemitting device changes with the lapse of time due to changes of theIel−Vel characteristic of the organic EL device 127. The change of theelectric potential appearing on the anode electrode of the organic ELdevice 127 is observed as a change of the gate-source voltage Vgs of thedriving transistor 121. In turn, the change of the gate-source voltageVgs of the driving transistor 121 causes a change in drain current or achange in driving current Ids. The change in driving current Ids causesa luminance change which varies from pixel to pixel and also varies inaccordance with the length of the lapsing time. As a result, the qualityof the image deteriorates.

<V-I Characteristic of the Driving Transistor>

In addition, if the characteristic of the driving transistor 121 variesfrom pixel to pixel, the characteristic variations have an effect on thedriving current Ids flowing through the driving transistor 121 as isobvious from Eq. (1). For example, if the mobility μ and/or thresholdvoltage Vth of the driving transistor 121 vary from pixel to pixel orchange with the lapse of time, the driving current Ids flowing throughthe driving transistor 121 also varies from pixel to pixel or changeswith the lapse of time even if the gate-source voltage Vgs is sustainedat a constant magnitude. As a result, the luminance of light emitted bythe organic EL device 127 also varies from pixel to pixel or changeswith the lapse of time.

For example, the characteristic of the driving transistor 121 employedin the pixel circuit P may vary from pixel to pixel due to variations ofthe process of manufacturing the driving transistor 121. In turn, thevariations in characteristic give rise to variations in drain current orvariations in driving current Ids from pixel to pixel even if thedriving transistor 121 is operating in a saturated region and even ifthe gate-source voltage Vgs is sustained at a magnitude uniform for allthe pixel circuits P. The variations in drain current or variations indriving current Ids appear as variations of the luminance of lightemitted by the organic EL device 127 from pixel to pixel. It is to benoted that the characteristic of the driving transistor 121 includes thecharacteristics the mobility μ and threshold voltage Vth of the drivingtransistor 121.

As described earlier, the driving current Ids (or the drain current) ofthe driving transistor 121 operating in a saturated region is expressedby Eq. (1). The threshold voltage Vth of the driving transistor 121 mayvary from pixel to pixel. If the threshold voltage Vth of the drivingtransistor 121 varies from pixel to pixel, the driving current Ids alsovaries from pixel to pixel as is obvious from Eq. (1) even if thegate-source voltage Vgs is sustained at a magnitude uniform for all thepixel circuits P. That is to say, unless a countermeasure against thevariations in threshold voltage Vth from pixel to pixel is taken, thedriving current Ids1 generated by a driving transistor 121 which has athreshold voltage of Vth1 and receives a gate-source voltage Vgs will beundesirably different from the driving current Ids2 generated by anotherdriving transistor 121 which has a threshold voltage of Vth2 differentfrom the threshold voltage of Vth1 and receives the same gate-sourcevoltage Vgs.

By the same token, the mobility μ of the driving transistor 121 may varyfrom pixel to pixel. If the mobility μ of the driving transistor 121varies from pixel to pixel, the driving current Ids also varies frompixel to pixel as is obvious from Eq. (1) even if the gate-sourcevoltage Vgs is sustained at a magnitude uniform for all the pixelcircuits P. That is to say, unless a countermeasure against thevariations in mobility μ from pixel to pixel is taken, the drivingcurrent Ids1 generated by a driving transistor 121 which has a mobilityof μ1 and receives a gate-source voltage Vgs will be undesirablydifferent from the driving current Ids2 generated by another drivingtransistor 121 which has a mobility of μ2 different from the mobility ofμ1 and receives the same gate-source voltage Vgs.

If the Vin-Ids characteristic varies much due to large variations inthreshold voltage Vth and/or mobility μ from pixel to pixel wherereference notation Vin denotes the high-level electric potential of avideo signal Vsig applied to the gate electrode G of the drivingtransistor 121, the driving current Ids and, thus, the luminance oflight emitted by the organic EL device 127 also vary as well from pixelto pixel even if the same signal amplitude ΔVin is applied to drivingtransistors 121 of the pixel circuits P. As a result, screen-luminanceuniformity cannot be obtained. This is because, if the threshold voltageVth and/or mobility μ of the driving transistor 121 employed in thepixel circuit P vary from pixel to pixel, the driving current Ids alsovaries from pixel to pixel in accordance with Eq. (1) and the luminanceof the emitted light also varies from pixel to pixel.

<Concept of Threshold-Voltage and Mobility Compensations>

By setting timings to carry out a threshold-voltage compensationfunction and a mobility compensation function as counter measuresagainst the variations in threshold voltage and variations in mobilityrespectively, effects of the variations can be repressed. Thus,screen-luminance uniformity can be obtained. The timings to carry out athreshold-voltage compensation function and a mobility compensationfunction will be described later in detail.

In a threshold-voltage compensation operation and a mobilitycompensation operation, which are carried out in accordance with thisembodiment, a write gain to be described in detail later can be assumedto have an ideal value of 1. With the write gain assumed to have anideal value of 1, the gate-source voltage Vgs applied to the drivingtransistor 121 in a light emission period is expressed by an expressionof “Vin+Vth−ΔV” where reference notation Vin denotes the high-levelelectric potential of the video signal Vsig as described above whereasreference notations Vth and ΔV used in the expression denote parametersof the threshold-voltage compensation operation and the mobilitycompensation operation respectively. The magnitude of the parameter Vthof the threshold-voltage compensation operation is equal to themagnitude of the threshold voltage Vth of the driving transistor 121.The parameter Vth of the threshold-voltage compensation operation is aparameter for compensating the driving transistor 121 for variations ofthe threshold voltage Vth of the driving transistor 121. On the otherhand, serving as a parameter for compensating the driving transistor 121for variations of the mobility μ of the driving transistor 121, theparameter ΔV of the mobility compensation operation has a magnitude setas described below. By carrying out the threshold-voltage compensationoperation and the mobility compensation operation, the driving currentIds can be made independent of variations and changes in thresholdvoltage Vth as well as variations and changes in mobility μ. As aresult, even if the threshold voltage Vth and/or the mobility μ vary dueto variations in manufacturing process or change with the lapse of time,the driving current Ids can be sustained at a constant magnitude, givena constant gate-source voltage Vgs applied to the driving transistor121, so that the luminance of light emitted by the organic EL device 127can also be sustained at a constant level.

The mobility compensation process is carried out to increase theparameter ΔV of the mobility compensation operation to a large value ΔV1for a driving transistor 121 having a large mobility μ1 or decrease theparameter ΔV of the mobility compensation operation to a small value ΔV2for a driving transistor 121 having a small mobility μ2. Thus, theparameter ΔV of the mobility compensation operation is used as anegative-feedback quantity proportional to the mobility μ of the drivingtransistor 121. For this reason, the parameter ΔV of the mobilitycompensation operation is also referred to as a negative-feedbackquantity ΔV.

<Pixel Circuit According to the Embodiment>

FIG. 5 is a diagram showing the pixel circuit P according to thisembodiment. It is to be noted that the diagram of FIG. 5 also shows thewire scanning section 104 and the driving scanning section 105 of thevertical-direction driving section 103 and the horizontal-directiondriving section 106, which are located on peripheries of the pixel arraysection 102 placed on the substrate 101 of the display panel section100. The configuration of the pixel circuit P according to thisembodiment is based on an Nch-type FET serving as the driving transistor121. In addition, the pixel circuit P according to this embodiment alsoincludes a first driving-signal stabilization circuit for repressingchanges caused by deteriorations of the organic EL device 127 with thelapse of time to appear as changes of the driving current Ids flowing tothe organic EL device 127. The first driving-signal stabilizationcircuit is a circuit for compensating the pixel circuit P for changesoccurring with the lapse of time to appear as changes of thecurrent-voltage characteristic of the organic EL device 127 serving as atypical electro-optical device in order to sustain the driving currentIds at a constant magnitude, given a constant gate-source voltage Vgsapplied to the driving transistor 121.

The first driving-signal stabilizing circuit adopts a driving method forsustaining the driving current Ids at a constant magnitude, given aconstant gate-source voltage Vgs applied to the driving transistor 121,by carrying out a threshold-voltage compensation and a mobilitycompensation function in order to get rid of driving-current variationscaused by variations of the characteristic of the driving transistor121. The variations of the characteristic of the driving transistor 121are variations of the threshold voltage and mobility of the drivingtransistor 121. In accordance with the driving method for repressingeffects exhibited by the driving current Ids as effects caused byvariations of the characteristic of the driving transistor 121, that is,variations of the threshold voltage and mobility of the drivingtransistor 121, while the pixel circuit P having the 2TR-drivenconfiguration is being used as it is to serve as the firstdriving-signal stabilizing circuit, a countermeasure against thevariations of the characteristic of the driving transistor 121 is takenby devising timings to drive the driving transistor 121 and the samplingtransistor 125. Since the 2TR-driven configuration is simple and thenumber of devices as well as the number of wires are small, the finenessof the pixel circuit P can be improved and, in addition to the improvedfineness, the video signal Vsig can be sampled without causingdeteriorations. Thus, a good quality of the image can be obtained.

The wiring connections of the signal holding capacitor 120 in the pixelcircuit P according to the embodiment are different from those of thepixel circuit P in the second typical comparison configuration. That isto say, the signal holding capacitor 120 is connected to othercomponents in the pixel circuit P according to the embodiment so as toform a bootstrap circuit functioning as a typical second driving-signalstabilization circuit. The bootstrap circuit functioning as a typicalsecond driving-signal stabilization circuit is a circuit for getting ridof driving-current changes caused by aging deteriorations. In otherwords, the signal holding capacitor 120 is wired in the pixel circuit Paccording to the embodiment so as to carry out the functions of thebootstrap circuit serving as the typical second driving-signalstabilization circuit for sustaining the driving current Ids at aconstant magnitude, even if the current-voltage characteristic of theorganic EL device 127 changes with the lapse of time.

To put it concretely, as shown in the diagram of FIG. 5, the pixelcircuit P according to the embodiment employs the Nch-type drivingtransistor 121, the Nch-type sampling transistor 125 and the organic ELdevice 127 serving as a typical electro-optical device which emits lightwhen a driving current Ids is flowing thereto. Since the organic ELdevice 127 generally has a rectification characteristic, the organic ELdevice 127 is shown by making use of the symbol of a diode. It is to benoted that the organic EL device 127 has a parasitic capacitor Cel. Inthe diagram of FIG. 5, the parasitic capacitor Cel is shown as acapacitor connected in parallel to the organic EL device 127 which isrepresented by the symbol of a diode.

The signal holding capacitor 120 is connected between the source andgate electrodes of the driving transistor 121. The source and gateelectrodes of the driving transistor 121 serve as nodes ND121 and ND122respectively. The source electrode of the driving transistor 121 is alsoconnected to the anode electrode A of the organic EL device 127. Thesignal holding capacitor 120 also serves as a bootstrap capacitor. Acathode electric potential Vcath used as a reference electric potentialis supplied to the cathode electrode K of the organic EL device 127. Thecathode electric potential Vcath used as a reference electric potentialis asserted on a cathode wire Wcath which serves the ground line GNDcommon to all pixel circuits P.

The gate electrode of the sampling transistor 125 is connected to thewrite scanning section 104 by a write scanning line 104WS whereas thedrain electrode of the sampling transistor 125 is connected to thehorizontal-line driving section 106 by a video-signal line 106HS. Thesource electrode of the sampling transistor 125 is connected to the gateelectrode of the driving transistor 121. As described above, the gateelectrode of the driving transistor 121 serves as the node ND122. Thewrite scanning section 104 supplies an active-high write driving pulseWS to the gate electrode of the sampling transistor 125. The samplingtransistor 125 can be connected by swapping the source and drainelectrodes with each other. In addition, the sampling transistor 125 canbe used as a depletion-type transistor or an enhancement-typetransistor.

The drain electrode of the driving transistor 121 is connected to thedriving scanning section 105 by a power-supply providing line 105DSL.The power-supply providing line 105DSL is a line which has a capabilityof supplying a power generated by the driving scanning section 105 tothe driving transistor 121. To put it concretely, the driving scanningsection 105 employs a power-supply voltage switching circuit forswitching a power-supply driving pulse DSL supplied to the drainelectrode of the driving transistor 121 from a high-level first electricpotential Vcc to a low-level second electric potential Vss or viceversa. In the following description, the second electric potential Vssis also referred to as a initialization electric potential Vini. Inactuality, a power-supply driving pulse DSL supplied to the drainelectrode of the driving transistor 121 through the power-supplyproviding line 105DSL to serve as a pulse for driving the drainelectrode has two levels, that is, a high level set at the firstelectric potential Vcc and a low level set at the second electricpotential Vss. By supplying the second electric potential Vss to thedrain electrode of the driving transistor 121, it is possible to drivethe pixel circuit P to carry out a preparatory operation preparing for athreshold-voltage compensation operation prior to the threshold-voltagecompensation operation.

The second electric potential Vss is an electric potential sufficientlylower than an aforementioned offset electric potential Vofs which is areference electric potential of a video signal Vsig appearing on thevideo-signal line 106HS. To put it concretely, the low-level secondelectric potential Vss appearing on the power-supply providing line105DSL is set at such a value that the gate-source voltage Vgs of thedriving transistor 121 is greater than the threshold voltage Vth of thedriving transistor 121. The gate-source voltage Vgs is a difference inelectric potential between the gate and source electrodes of the drivingtransistor 121. It is to be noted that the offset electric potentialVofs is applied to the video-signal line 106HS during an initializationoperation prior to a threshold-voltage compensation operation and alsoused for pre-charging the video-signal line 106HS in advance.

In the pixel circuit P described above, in an operation to drive theorganic EL device 127, the first electric potential Vcc is supplied tothe drain electrode of the driving transistor 121 and, since the sourceelectrode S of the driving transistor 121 is connected to the anodeelectrode of the organic EL device 127, as a whole, the drivingtransistor 121 and the organic EL device 127 form a source follower.

The pixel circuit P described above adopts the so-called 2TR drivingconfiguration employing the driving transistor 121 and the switchingtransistor 125 (or the sampling transistor 125) used for scanningoperations. In addition, by properly setting timings of the power-supplydriving pulse DSL for supplying the first electric potential Vcc or thesecond electric potential Vss to the driving transistor 121 as well astimings of the write driving pulse WS to put the sampling transistor 125in a turned-on and turned-off states, it is possible to get rid ofeffects of aging deteriorations of the organic EL device 127 and/oreffects of variations of the characteristic of the driving transistor121 on the driving current Ids. As described earlier, the variations ofthe characteristic of the driving transistor 121 are typicallyvariations of the threshold voltage and mobility of the drivingtransistor 121.

The write scanning section 104, the driving scanning section 105 and thehorizontal-direction driving section 106 are provided on peripheries ofthe pixel array section 102 to serve as sections for driving the pixelcircuit P. The write scanning section 104, the driving scanning section105 and the horizontal-direction driving section 106 form a controlsection 109. The control section 109 functions as a driving-signalstabilization circuit for properly setting driving timings so as tosustain the driving current Ids generated by the driving transistor 121at a constant magnitude, given a constant gate-source voltage Vgsapplied to the driving transistor 121. To put it concretely, first ofall, it is desirable to execute control of operating the write scanningsection 104 to put the sampling transistor 125 in a turned-off state inorder to stop an operation to supply the video signal Vsig to the gateelectrode (that is, the control input terminal) of the drivingtransistor 121 at a point of time information corresponding to thesignal amplitude ΔVin has been stored in the signal holding capacitor120. With the sampling transistor 125 put in a turned-off state, abootstrap operation takes place by virtue of the coupling effect of thesignal holding capacitor 120 as an operation in which the electricpotential appearing on the control input terminal of the drivingtransistor 121 changes in a manner of being interlocked with theelectric potential appearing on the source gate (that is, the outputterminal of the driving transistor 121).

It is desirable to operate the control section 109 to execute controlleading to the bootstrap operation also in an initial part of a lightemission start lagging behind the end of the sampling operation. That isto say, in a state of supplying the high-level electric potential Vin ofthe video signal Vsig to the sampling transistor 125, the samplingtransistor 125 is put in a turned-on state and then put in a turned-offstate so that the difference in electric potential between the controlinput terminal and output terminal of the driving transistor 121 issustained at a constant magnitude.

In addition, it is desirable to operate the control section 109 toexecute control leading to execution of a bootstrap operation in a lightemission period shown in the timing diagram of FIG. 6 as a period (G) sothat an operation to compensate the pixel circuit P for agingdeteriorations of the characteristic the organic EL device 127 servingas an electro-optical device is implemented by execution of thebootstrap operation. In order to implement such a compensation period,in a period during which the driving current Ids according toinformation stored in the signal holding capacitor 120 is flowing to theorganic EL device 127 serving as an electro-optical device, the samplingtransistor 125 is put in a continuous turned-off state in advance sothat the difference in electric potential between the control inputterminal and output terminal of the driving transistor 121 can besustained at a constant magnitude in order to implement the operation tocompensate the pixel circuit P for aging deteriorations of thecharacteristic the organic EL device 127. By letting the bootstrapoperation take place in a light emission period as the bootstrapoperation based on the coupling effect of the signal holding capacitor120, the difference in electric potential between the control inputterminal and output terminal of the driving transistor 121 can besustained at a constant magnitude by virtue of the bootstrap operationbased on the coupling effect of the signal holding capacitor 120 even ifthe current-voltage characteristic of the organic EL device 127 changeswith the lapse of time. Thus, the luminance of light emitted by theorganic EL device 127 can be sustained at a constant magnitude all thetime.

On top of that, it is desirable to operate the control section 109 toexecute control of carrying out a threshold-voltage compensationoperation for holding a voltage corresponding to the threshold voltageVth of the driving transistor 121 in the signal holding capacitor 120 byputting the sampling transistor 125 in a turned-on state during a timeband in which the offset electric potential Vofs is being supplied tothe input terminal of the sampling transistor 125. Typically, the sourceelectrode of the sampling transistor 125 is used as the input terminalof the sampling transistor 125. If necessary, the threshold-voltagecompensation operation is carried out repeatedly in a plurality ofhorizontal periods leading ahead of an operation to store informationcorresponding to the signal amplitude ΔVin in the signal holdingcapacitor 120. By carrying out the threshold-voltage compensationoperation repeatedly, the voltage corresponding to the threshold voltageVth of the driving transistor 121 can be held in the signal holdingcapacitor 120 with a high degree of reliability.

In addition, it is desirable to operate the control section 109 toexecute control of putting the sampling transistor 125 in a turned-onstate in order to carry out a preparatory operation preparing for athreshold-voltage compensation operation during a time band, in whichthe offset electric potential Vofs is being supplied to the inputterminal of the sampling transistor 125, prior to the threshold-voltagecompensation operation. The preparatory operation preparing for athreshold-voltage compensation operation includes a dischargingoperation and an initialization operation. That is to say, prior to thethreshold-voltage compensation operation, an electric potentialappearing between the control input terminal and output terminal of thedriving transistor 121 is initialized. To put it in detail, a differencein electric potential between the terminals of the signal holdingcapacitor 120 which is connected between the control input terminal andoutput terminal of the driving transistor 121 is set at a level at leastequal to the threshold voltage Vth of the driving transistor 121.

<Operations of the Pixel Circuit According to the Embodiment>

FIG. 6 is a timing diagram showing a plurality of explanatory timingcharts referred to in description of basic driving timings of theembodiment shown in the diagram of FIG. 5 as an embodiment implementingthe pixel circuit P. Each of FIGS. 7A to 7G is an explanatory circuitdiagram referred to in description of an equivalent circuit of the pixelcircuit P and the operation of the equivalent circuit in one of periods(A) to (G) shown at the bottom of the timing diagram of FIG. 6.

The timing diagram of FIG. 6 has the horizontal axis serving as a timeaxis common to all the timing charts. The timing charts of the timingdiagram of FIG. 6 show electric-potential changes of the write drivingpulse WS appearing on the write driving line 104WS, the power-supplydriving pulse DSL appearing on the power-supply providing line 105DSLand the video signal Vsig appearing on the video-signal line 106HS. Inaddition, the timing diagram of FIG. 6 also shows changes of the gatevoltage Vg appearing on the gate electrode of every driving transistor121 on a pixel row and the source voltage Vs appearing on the sourceelectrode of the driving transistor 121. The changes of the gate voltageVg and the source voltage Vs occur in parallel to the electric-potentialchanges of the write driving pulse WS, the power-supply driving pulseDSL and the video signal Vsig.

In the following description, unless otherwise specified, the write gaindescribed below is assumed to have an ideal value of 1 in order to makethe explanation easy to understand. Thus, the following descriptionexplains that information corresponding to the magnitude of the signalamplitude ΔVin is written, stored or held in the signal holdingcapacitor 120. If the write gain is smaller than the ideal value of 1,information with an amount smaller than the magnitude of the signalamplitude ΔVin is written, stored or held in the signal holdingcapacitor 120. That is to say, information corresponding to a productobtained as a result of multiplying the magnitude of the signalamplitude ΔVin by the write gain is written, stored or held in thesignal holding capacitor 120.

By the way, reference notation Ginput is used in this patentspecification to denote the write gain which is defined as a ratio ofthe amount of information actually stored in the signal holdingcapacitor 120 to the signal amplitude ΔVin. The write gain Ginput isexplained concretely as follows. Instead of being stored merely in thesignal holding capacitor 120, the signal amplitude ΔVin is actually heldin a capacitor series circuit. The capacitor series circuit includes atotal capacitor C1 and a capacitor C2 which are connected to each otherin series. The total capacitor C1 represents the signal holdingcapacitor 120 and a parasitic capacitor which forms a parallel circuitin conjunction with the signal holding capacitor 120. On the other hand,the parasitic capacitor C2 is a parasitic capacitor which forms a seriescircuit in conjunction with the signal holding capacitor 120. The writegain Ginput is expressed as follows:Ginput=C2/(C1+C2)=1−C1/(C1+C2)=1−gIn the above equation, reference notation g denotes a quantity expressedas follows:g=C1/(C1+C2)

In addition, in the following simplified description, unless otherwisespecified, a bootstrap gain Gbst is assumed to also have an ideal valueof 1 in order to make the explanation easy to understand. In a pixelcircuit P including the driving transistor 121 and the signal holdingcapacitor 120 connected between the gate and source electrodes of thedriving transistor 121, with the sampling transistor 125 put in aturned-off state, a bootstrap operation may take place as an operationin which the electric potential appearing on the gate electrode of thedriving transistor 121 changes in a manner of being interlocked with theelectric potential appearing on the source gate of the drivingtransistor 121 due to the coupling effect of the signal holdingcapacitor 120. The bootstrap gain Gbst is defined as the ratio of anincrease of the gate voltage Vg appearing on the gate electrode in abootstrap operation to an increase of the source voltage Vs appearing onthe source electrode in the same bootstrap operation. Thus, thebootstrap gain Gbst indicates the power of the bootstrap operation. Toput it concretely, the bootstrap gain Gbst can be expressed by thefollowing equation:Gbst=(Cs+Cgs)/(Cs+Cgs+Cgd+Cws)

In the above equation, reference notation Cs denotes the capacitance ofthe signal holding capacitor 120, reference notation Cgs denotes thecapacitance of a parasitic capacitor C121 gs existing between the gateand source electrodes of the driving transistor 121, reference notationCgd denotes the capacitance of a parasitic capacitor C121 gd existingbetween the gate and drain electrodes of the driving transistor 121 andreference notation Cws denotes the capacitance of a parasitic capacitorC125 gs existing between the gate and source electrodes of the samplingtransistor 125.

Basically, the write scanning line 104WS is subjected to the sameoperation periodically at intervals each equal to one horizontalscanning period denoted by reference notation 1H in the timing diagramof FIG. 6. By the same token, the power-supply providing line 105DSL isalso subjected to the same operation periodically at intervals eachequal to one horizontal scanning period. The early part of thehorizontal scanning period 1H is the so-called an ineffective period ofthe video signal Vsig whereas the later part of the horizontal scanningperiod 1H is the so-called an effective period of the video signal Vsig.With timings according to the embodiment, the video signal Vsig assertedon the video-signal line 106HS is set at the offset electric potentialVofs in the ineffective period and set at the high-level electricpotential Vin (=Vofs+ΔVin) in the effective period where referencenotation ΔVin denotes the signal amplitude mentioned before. In thisembodiment, the one horizontal period is taken as a processing cycle inwhich the threshold-voltage compensation operation is carried out once.In actuality, however, the threshold-voltage compensation operation canbe carried out a plurality of times.

The one horizontal period becomes a processing cycle of thethreshold-voltage compensation operation because, during the onehorizontal period, for every pixel row, prior to the threshold-voltagecompensation operation which is carried out before the samplingtransistor 125 samples information corresponding to the signal amplitudeΔV and stores (or writes) the sampled information in the signal holdingcapacitor 120, an initialization operation is performed. In theinitialization operation, the power-supply driving pulse DSL asserted onpower-supply providing line 105DSL is set at the second electricpotential Vss and the voltage appearing on the gate electrode of thedriving transistor 121 is set at the offset electric potential Vofs. Theelectric potential appearing on the source electrode of the drivingtransistor 121 is also set at the second electric potential Vss. Then,in a state of setting the power-supply driving pulse DSL asserted onpower-supply providing line 105DSL at the first electric potential Vccand during a time band in which the offset electric potential Vofs isappearing on the video-signal line 106HS, the sampling transistor isconducted. The threshold-voltage compensation operation is carried outin order to hold a voltage corresponding to the threshold voltage Vth ofthe driving transistor 121.

Since the period (D) shown in the timing diagram of FIG. 6 as the periodof the threshold-voltage compensation operation is terminated before theend of the one horizontal period, the period of the threshold-voltagecompensation operation is naturally shorter than the one horizontalperiod. Thus, there may be a case in which the voltage accuratelycorresponding to the threshold voltage Vth of the driving transistor 121cannot be held in the signal holding capacitor 120 by carrying out inthe short period of the threshold-voltage compensation operation forsome reasons including a reason relevant to a relation between thecapacitance Cs of the signal holding capacitor 120 and the magnitude ofthe second electric potential Vss. In order to solve this problem, thethreshold-voltage compensation operation is carried out a plurality oftimes in this embodiment. That is to say, in each of a plurality ofhorizontal periods leading ahead of the operation to sample theinformation corresponding to the signal amplitude ΔVin and store (orwrite) the sampled information in the signal holding capacitor 120, thethreshold-voltage compensation operation is carried out. Thus, thethreshold-voltage compensation operation is carried out a plurality oftimes in the same plurality of respective horizontal periods leadingahead of the information sampling and storing operation in order to holdthe voltage corresponding to the threshold voltage Vth of the drivingtransistor 121 in the signal holding capacitor 120 with a high degree ofreliability.

First of all, the following description starts with an explanation ofthe light emission period in which the power-supply driving pulse DSL isset at the first electric potential Vcc, the video signal Vsig is set atthe high-level electric potential Vin and the sampling transistor 125 isput in a turned-off state as shown in a circuit diagram of FIG. 7A bysetting the write driving pulse WS at the low level WS_L. In the lightemission period, the driving transistor 121 is set to operate in asaturated region. Thus, the driving current Ids flowing from the drivingtransistor 121 to the organic EL device 127 has a magnitude determinedby the gate-source voltage Vgs of the driving transistor 121 inaccordance with Eq. (1).

Then, when the power-supply driving pulse DSL asserted on thepower-supply providing line 105DSL is changed from the first electricpotential Vcc to the second electric potential Vss also referred to asthe initialization electric potential Vini as shown in a circuit diagramof FIG. 7B, the power-supply driving pulse DSL becomes the source of thedriving transistor 121 and the source voltage Vs of the drivingtransistor 121 becomes all but equal to the initialization electricpotential Vini which satisfies the following relation of Vini(<Vth(el)+Vcath) where reference notation Vth(el) denotes the thresholdvoltage of the organic EL device 127 and reference notation Vcathdenotes the cathode electric potential Vcath. With the source voltage Vsof the driving transistor 121 becoming all but equal to theinitialization electric potential Vini, the organic EL device 127 ceasesto emit light.

As shown in a circuit diagram of FIG. 7C, when the write driving pulseWS asserted on the write scanning line 104WS is changed from the lowlevel WS_L to the high level WS_H in order to put the samplingtransistor 125 in a turned-on state, the offset electric potential Vofsis supplied to the gate electrode of the driving transistor 121 and thesource voltage Vs of the driving transistor 121 becomes equal to theinitialization electric potential Vini as shown in a circuit diagram ofFIG. 7C. Thus, at that time, the gate-source voltage Vgs of the drivingtransistor 121 becomes equal to (Vofs−Vini). If (Vofs−Vini) is notgreater than the threshold voltage Vth of the driving transistor 121,the threshold-voltage compensation operation cannot be carried out. Itis thus necessary to set the voltages at such values that the relationof (Vofs−Vini)>Vth is satisfied. In the period (C) shown in the diagramof FIG. 6 as a period corresponding to the circuit diagram of FIG. 7C,the gate electric potential Vg appearing on the gate electrode of thedriving transistor 121 is established at the offset electric potentialVofs and the source electric potential Vs appearing on the sourceelectrode of the driving transistor 121 is established at theinitialization electric potential Vini so that the gate-source voltageVgs (=Vofs−Vini) is greater than the threshold voltage Vth of thedriving transistor 121. The period (C) is referred to as theinitialization operation described above.

Then, the power-supply driving pulse DSL appearing on the power-supplyproviding line 105DSL is changed from the initialization electricpotential Vini to the first electric potential Vcc as shown in a circuitdiagram of FIG. 7D, putting the driving transistor 121 in a turned-onstate. With the driving transistor 121 put in a turned-on state, thedriving current Ids is flowing through the driving transistor 121,electrically charging the parasitic capacitor Cel of the organic ELdevice 127. Thus, the source voltage Vs of the driving transistor 121rises and, after the lapse of time determined in advance, thegate-source voltage Vgs of the driving transistor 121 becomes equal tothe threshold voltage Vth of the driving transistor 121, putting thedriving transistor 121 in a turned-off state. This operation to set thedriving transistor 121 at the threshold voltage Vth of the drivingtransistor 121 is referred to as the threshold-voltage compensationoperation.

The write driving pulse WS appearing on the write scanning line 104WS ischanged from the high level WS_H to the low level WS_L in order to putthe sampling transistor 125 in a turned-off state as shown in a circuitdiagram of FIG. 7E. With the sampling transistor 125 put in a turned-offstate, the gate electric potential Vg appearing on the gate electrode ofthe driving transistor 121 is put in a floating state but, since thegate-source voltage Vgs of the driving transistor 121 is equal to thethreshold voltage Vth, the driving transistor 121 remains in aturned-off state, disallowing the driving current Ids to flow.

The period (F) serving as the signal electric-potential write period/amobility compensation period is started when the write driving pulse WSasserted on the write scanning line 104WS is changed from the low levelWS_L to the high level WS_H in order to put the sampling transistor 125in a turned-on state as shown in a circuit diagram of FIG. 7F. As shownin a circuit diagram of FIG. 7F, during the period (F), the video signalVsig asserted on the video-signal line 106HS has been changed from theoffset electric potential Vofs to the high-level electric potential Vin,which is equal to (Vofs+ΔVin), as described above. Thus, an electricpotential corresponding to ΔVin is added to the gate voltage Vg (or thegate-source voltage Vgs) appearing on the gate electrode of the drivingtransistor 121 to be stored in the signal holding capacitor 120 in theso-called signal electric-potential write operation. Since thegate-source voltage Vgs has been set at the threshold voltage Vth of thedriving transistor 121 in the threshold-voltage compensation operationcarried out in the period (D) as described above, the gate-sourcevoltage Vgs becomes equal to (ΔVin+Vth) which is greater than thethreshold voltage Vth of the driving transistor 121. In addition, thedriving transistor 121 is put in a turned-on state whereas the drivingcurrent Ids flows from the power-supply providing line 105DSL to thesignal holding capacitor 120 having a capacitance Cs and the parasiticcapacitor Cel included in the organic EL device 127 as a capacitorhaving a capacitance Cel by way of the driving transistor 121. Thus, thesource voltage Vs of the driving transistor 121 rises with the lapse oftime.

Before supplying the high-level electric potential Vin to the gateelectrode of the driving transistor 121 in the period (F), thegate-source voltage Vgs has been set at the threshold voltage Vth inadvance. The driving current Ids flowing through the driving transistor121 may reflect variations of the mobility μ of the driving transistor121. To put it in detail, for a driving transistor 121 having a largemobility μ, the driving current Ids has a large magnitude and the sourcevoltage Vs rises fast. For a driving transistor 121 having a smallmobility μ, on the other hand, the driving current Ids has a smallmagnitude and the source voltage Vs rises slowly. The source voltage Vsof the driving transistor 121 rises by an electric-potential differenceΔV to reduce the gate-source voltage Vgs in a direction of compensatingthe driving transistor 121 for variations in mobility μ. After the lapseof certain time, the resulting gate-source voltage Vgs is approximatelya voltage obtained as a result of the so-called mobility compensationoperation of compensating the driving transistor 121 for variations inmobility μ. The period (F) is thus also the period of the mobilitycompensation operation of the driving transistor 121.

The write driving pulse WS asserted on the write scanning line 104WS ischanged from the high level WS_H to the low level WS_L in order to putthe sampling transistor 125 in a turned-off state and to make atransition to a period (G) serving as a light emission period in whichthe pixel circuit P is put in an operating state shown in the circuitdiagram of FIG. 7G. Since the gate-source voltage Vgs of(Vin−Vofs+Vth−ΔV) is greater than the threshold voltage Vth of thedriving transistor 121, that is, the relation of (Vin−Vofs+Vth−ΔV)>Vthholds true, the driving current Ids is flows, further raising the sourcevoltage Vs of the driving transistor 121. Since the sampling transistor125 has been put in a turned-off state causing the gate electrode of thedriving transistor 121 to enter a floating state, however, a bootstrapoperation occurs due to the coupling effect of the signal holdingcapacitor 120 while the source voltage Vs of the driving transistor 121is rising. Thus, the gate voltage Vg of the driving transistor 121 isalso rising in a manner of being interlocked with the source voltage Vsin the bootstrap operation. As a result, the gate-source voltage Vgs ofthe driving transistor 121 is sustained at (Vin−Vofs+Vth−ΔV), keeping astate of outputting a constant driving current Ids to the organic ELdevice 127 so that the organic EL device 127 emits light.

The driving current Ids can be expressed by Eq. (2) described below toserve as a relation between the driving current Ids and the gate-sourcevoltage Vgs. Eq. (2) is derived from Eq. (1) given earlier as anequation representing the characteristic of the driving transistor 121by assuming that the write gain has an ideal value of 1 and substitutingan expression of (Vin−ΔV+Vth) into the expression on the right-hand sideof Eq. (1) as a replacement for Vgs. Reference notation k used in theexpression on the right-hand side of Eq. (2) denotes a quantityexpressed as follows:k=(1/2)(W/L)CoxIds=kμ(Vgs−Vth)^2=kμ(ΔVin−ΔV)^2  (2)

The term of the threshold voltage Vth disappears from Eq. (2) clearlyindicating that the driving current Ids supplied to the organic ELdevice 127 is not dependent on the threshold voltage Vth of the drivingtransistor 121. Basically, the driving current Ids is determined by thehigh-level electric potential Vin. Strictly speaking, the drivingcurrent Ids is determined by the gate-source voltage Vgs which is avoltage stored in the signal holding capacitor 120 as a result of asampling operation to sample the signal amplitude ΔVin. In other words,the organic EL device 127 emits light with a luminance according to thesignal amplitude ΔVin. In the light emission state of the organic ELdevice 127, the voltage stored in the signal holding capacitor 120 asinformation determining the driving current Ids of the drivingtransistor 121 has also been corrected by the mobility compensationoperation parameter ΔV which is used as the negative-feedback quantitycited above. The parameter ΔV of the mobility compensation operationworks to just eliminate the effect of the mobility μ positioned at acoefficient term in the expression on the right-hand side of Eq. (2).Thus, the driving current Ids generated by the driving transistor 121 isessentially dependent only on the signal amplitude ΔVin.

That is to say, as a result of the operations carried out by the pixelcircuit P as described above, the gate-source voltage Vgs appearingbetween the gate and source electrodes of the driving transistor 121during the light emission period is determined by the expression of(Vin−Vofs+Vth−ΔV) which includes the term Vth serving as a parameter forcompensating the driving transistor 121 for variations of the thresholdvoltage Vth of the driving transistor 121 and includes the term ΔVserving as a parameter for compensating the driving transistor 121 forvariations of the mobility μ of the driving transistor 121. Theexpression of (Vin−Vofs+Vth−ΔV) is not affected by changes of the sourcevoltage Vs of the driving transistor 121, that is, changes of thevoltage appearing on the anode electrode of the organic EL device 127.This is because the gate voltage Vg of the driving transistor 121 alsochanges in a manner of being interlocked with the source voltage Vs ofthe driving transistor 121 during the bootstrap operation based on thecoupling effect of the signal holding capacitor 120. By the way, thechanges of the voltage appearing on the anode electrode of the organicEL device 127 are caused by changes of the I-V characteristic of theorganic EL device 127 with the lapse of time. Thus, it is possible toget rid of variations of the characteristic of the driving transistor121 and aging deteriorations of the organic EL device 127. That is tosay, by driving the pixel circuit P to carry out operations accurately,it is possible to get rid of the variations of the characteristic of thedriving transistor 121 and the aging deteriorations of the organic ELdevice 127.

<Problems>

FIGS. 8 and 9A to 9C are explanatory diagrams referred to in descriptionof problems raised by the display panel section 100 according to theembodiment. To be more specific, FIG. 8 is an explanatory diagramreferred to in description of a problem caused by the wire resistance ofthe power-supply providing line 105DSL serving as a power-supply line toappear as a problem raised in an operation to display an image at an allwhite display time. On the other hand, FIGS. 9A to 9C are explanatorydiagrams each referred to in description of a problem caused by the wireresistance of the power-supply providing line 105DSL serving as apower-supply line to appear as a problem raised in an operation todisplay an image at a window pattern display time. It is to be notedthat the explanatory diagrams of FIGS. 9A to 9C are also used fordescribing causes of the problems explained by referring to theexplanatory diagrams of FIGS. 8 and 9A to 9C as causes attributed to arelation with an early effect of the transistors.

As shown in the explanatory diagram of FIG. 8, each of scanning linesLscan is oriented in the horizontal direction. In the followingdescription with reference to the explanatory diagram of FIG. 8, thepower-supply providing line 105DSL used in the power-supplyvertical-direction scanning operation is taken as a typical scanningline Lscan. It is thus necessary to consider a problem caused by thewire resistance of the power-supply providing line 105DSL.

As shown in the diagram of FIG. 5, the pixel circuit P is configured toemploy two transistors, that is, the driving transistor 121 and thesampling transistor 125, as well as one capacitor, i.e., the signalholding capacitor 120. The pixel circuit P is driven to carry out athreshold-voltage compensation function, a mobility compensationfunction and a bootstrap function. In this case, the power-supplydriving pulse DSL asserted on the power-supply providing line 105DSLserving as a power-supply providing line connected to the drainelectrode of the driving transistor 121 is switched from the firstelectric potential Vcc to the second electric potential Vss alsoreferred to as the initialization electric potential Vini or vice versa.As shown in the explanatory diagrams of FIG. 8 and FIG. 9A, thepower-supply providing line 105DSL is oriented in the horizontaldirection and the driving current Ids of each of pixel circuits P on thesame pixel row as the power-supply providing line 105DSL flows to thecathode electric wire Wcath provided to serve as a line common to allpixel circuits P and to serve as a line for supplying a referenceelectric potential to all the pixel circuits P. Typically, the groundline GND is used as the cathode electric wire Vcath for supplying areference electric potential of 0 to all the pixel circuits P.

In the configuration described above, a voltage drop on the side closeto the control section 109 of the pixel array section 102 is smallerthan a voltage drop on the side opposite to the side close to thecontrol section 109 due to a difference in wire resistance between thescanning line Lscan stretched from the control section 109 to the pixelcircuit P on the side close to the control section 109 and the scanningline Lscan stretched from the control section 109 to the pixel circuit Pon the opposite side. The side close to the control section 109 isreferred to the scanning-signal input edge side which is the panelleft-edge side in the explanatory diagrams of FIGS. 8 and 9C. On theother hand, the side opposite to the side close to the control section109 is referred to the scanning-signal output edge side which is thepanel right-edge side in the explanatory diagrams of FIGS. 8 and 9C.Thus, the electric potential supplied to the drain electrode (that is,the power-supply providing terminal) of the driving transistor 121 onthe scanning-signal output edge side is lower than the electricpotential supplied to the drain electrode of the driving transistor 121on the scanning-signal input edge side. That is to say, the longer thedistance between the driving transistor 121 and the driving scanningsection 105, the larger the wire resistance and, thus, the larger thevoltage drop along the power-supply providing line 105DSL stretched tothe driving transistor 121. Therefore, the longer the distance betweenthe driving transistor 121 and the driving scanning section 105, thelower the electric potential supplied to the drain electrode (that is,the power-supply providing terminal) of the driving transistor 121. Inthe explanatory diagrams of FIGS. 8 and 9C, each of the wire resistancesis shown as the symbol of a resistor.

In such a configuration, as the distance between the driving transistor121 and the driving scanning section 105 gradually increases, thevoltage applied between the drain and source electrodes of the drivingtransistor 121 gradually decreases. Thus, the driving transistor 121 isaffected by the so-called early effect. If affected by the early effect,as the distance between the driving transistor 121 and the drivingscanning section 105 gradually increases, the driving current Idsgenerated by the driving transistor 121 gradually decreases even if themethod for getting rid of variations in characteristic as explainedearlier by referring to the diagrams of FIGS. 6 and 7A to 7G is adopted.

The early effect causes a phenomenon varying in accordance with thedisplay pattern to appear in the displayed image. FIG. 8 is a diagramreferred to in description of phenomena which occur in the case of anall white display. In the case of an all white display, as the distancebetween the driving transistor 121 and the driving scanning section 105gradually increases, the voltage drop along the power-supply providingline 105DSL stretched to the driving transistor 121 also graduallyincreases as well as described above. FIG. 9B is a diagram showing thecharacteristic of the driving transistor 121. As is obvious from thediagram of FIG. 9B, as the power-supply voltage appearing as an electricpotential supplied to the drain electrode of the driving transistor 121decreases, the driving current Ids representing the luminance of lightemitted by the organic EL device 127 gradually decreases. Thus, in thecase of an all white display, the luminance of light emitted by theorganic EL device 127 gradually decreases even if a mechanism applyingthe method for getting rid of variations in characteristic as explainedearlier by referring to the diagrams of FIGS. 6 and 7A to 7G is adopted.As a result, the luminance of light emitted by the organic EL device 127gradually decreases as shown in the block diagram of FIG. 8, giving riseto the so-called shading phenomenon.

In addition, if the driving current Ids flowing through the drivingtransistor 121 in accordance with a video pattern varies, thepower-supply voltage also changes from pixel circuit P to pixel circuitP along a pixel row oriented in the horizontal direction due to the wireresistance of the power-supply providing line 105DSL and the cathodeelectric wire Vcath which serve as power-supply lines. To put it indetail, the power-supply voltage is the drain-source voltage Vds appliedbetween the drain and source electrodes of the driving transistor 121.In particular, the driving current Ids flows to the cathode electrode ofthe organic EL device 127 in every pixel circuit P on the same pixelcolumn. Thus, the cathode electrode of every organic EL device 127 inevery pixel circuit P on the same pixel column is floated at anelectric-potential level higher than the ground electric potential GNDby an electric-potential difference equal to a product Rcath X Ids_allwhere reference notation Rcath denotes the wire resistance of thecathode electric wire Wcath and reference notation Ids_all denotes thetotal of driving currents Ids each flowing to the cathode electrode ofthe organic EL device 127 in one of pixel circuits P on the same pixelcolumn. As a result, the drain-source voltage Vds applied between thedrain and source electrodes of the driving transistor 121 also changesas well.

As is obvious from the diagram of FIG. 9B, as the drain-source voltageVds of the driving transistor 121 changes due to the early effect, thedriving current Ids of the driving transistor 121 also changes even ifthe driving transistor 121 is operating in the saturated region and thegate-source voltage Vgs of the driving transistor 121 is sustained at aconstant value. Typically, the changes of the driving current Ids of thedriving transistor 121 are eventually recognized as horizontal-directioncrosstalk in an operation to display a window pattern as shown in thediagram of FIG. 9C. As a typical countermeasure against thehorizontal-direction crosstalk, for example, it is necessary to get ridof current decreases due to voltage drops. In general, the level ofrecognition of differences in luminance is equal to or lower than 1%.Thus, a countermeasure needs to be taken against thehorizontal-direction crosstalk so as to satisfy the condition for thelevel of recognition of differences in luminance.

In addition, the state in which the cathode electric potential isfloated over the ground electric potential GND due to the total drivingcurrent Ids_all varies in accordance with the driving current Ids, thatis, in accordance with the gradation. Thus, the characteristicundesirably also varies from gradation to gradation as well. As aresult, in the case of a color display, it is feared that a color phaseshift exists.

The following description explains a mechanism provided in accordancewith an embodiment to serve as a mechanism for solving a variety ofproblems described above.

<Basic Principle>

FIG. 10 is an explanatory conceptual diagram referred to in descriptionof a mechanism provided in accordance with embodiments to serve as amechanism for repressing display irregularities caused by voltage dropsat a plurality of points along the power-supply providing line 105DSL.The embodiments are the first to third embodiments described later.

The basic concept of the mechanism according to the embodiment underliesa layout method for adjusting the length and/or width of a draw wire121DL which is also referred to as a drain wire which is connecting thedrain electrode of the driving transistor 121 employed in each pixelcircuit P to the power-supply providing line 105DSL. As mentionedpreviously, the drain electrode of the driving transistor 121 is alsoreferred to as the power-supply providing terminal of the drivingtransistor 121. In accordance with the layout method, at least one ofthe length and width of the draw wire 121DL are adjusted for each of thepixel circuits P so that the electric potentials each appearing on thedrain electrode (that is, the power-supply providing terminal) of thedriving transistor 121 employed in one of the pixel circuits P areuniform for the pixel circuits P under a condition of driving theorganic EL device 127 employed in each of pixel circuits P laid out inthe longitudinal direction of the power-supply providing line 105DSL toemit light with a uniform luminance. Thus, the electric potentials eachappearing on the drain electrode (that is, the power-supply providingterminal) of the driving transistor 121 employed in one of the pixelcircuits P are made extremely uniform for the pixel circuits P in orderto drive the organic EL device 127 employed in each of the pixelcircuits P to emit light with a uniform luminance. As a result, it ispossible to get rid of luminance irregularities caused by voltage dropsat a plurality of points along the power-supply providing line 105DSL toappear as irregularities such as shading and crosstalk.

To put it concretely, first of all, the power-supply providing line105DSL is laid out as a line having portions made as thick as possibleas a prerequisite. The portions made as thick as possible do not includeportions crossing other wires because it is difficult to make theportions crossing other wires thick. By making the portions except theportions crossing other wires as thick as possible, it is possible tomuch reduce voltage drops caused by the wire resistance of thepower-supply providing line 105DSL itself. On top of that, the lengthand/or width of the draw wire 121DL are adjusted for each of the pixelcircuits P laid out in the longitudinal direction of the power-supplyproviding line 105DSL so that the sum of the wire resistance of the drawwire 121DL provided for any specific one of the pixel circuits P and thewire resistance of the power-supply providing line 105DSL connecting thespecific pixel circuit P to the driving scanning section 105 becomesuniform for all the pixel circuits P. That is to say, the varyingvoltage drops at a plurality of points along the power-supply providingline 105DSL can be compensated for by the varying wire resistances ofthe draw wires 121DL.

Basically, the wire resistance of the draw wire 121DL provided for apixel circuit P separated away from the driving scanning section 105serving as a source generating the power-supply driving pulse DSL (orthe power-supply voltage having a pulse shape) by a short longitudinaldirection distance is made relatively large whereas the wire resistanceof the draw wire 121DL provided for a pixel circuit P separated awayfrom the driving scanning section 105 serving as a source generating thepower-supply driving pulse DSL by a long longitudinal direction distanceis made relatively small. The longitudinal direction distance is adistance in the longitudinal direction of the power-supply providingline 105DSL. By setting the wire resistance of the draw wire 121DLprovided for the pixel circuit P at such a value that, the longer (orshorter) the longitudinal direction distance between the drivingscanning section 105 and the pixel circuit P, the smaller (or larger)the wire resistance, the varying voltage drops at a plurality of pointsalong the power-supply providing line 105DSL in the display panelsection 100 can be compensated for by the varying wire resistances ofthe draw wires 121DL. Thus, the generation of the shading and/or thecrosstalk can be avoided. As a result, it is possible to implement adisplay apparatus having a good image quality.

FIG. 10 is a block diagram showing a typical configuration including amatrix of pixel circuits P embedded in the pixel array section 102 as apixel matrix having two horizontal-direction pixel rows and threevertical-direction pixel columns. As shown in the block diagram of FIG.10, pixel circuits P1, P2 and P3 are provided on the first pixel row.The pixel circuit P1 is a pixel matrix P on the side close to thedriving scanning section 105, the pixel circuit P2 is a pixel matrix Pat the middle of the pixel row and the pixel circuit P3 is a pixelcircuit P on the side close to the right edge of the display panelsection 100. By the same token, pixel circuits P4, P5 and P6 areprovided on the second pixel row. The pixel circuit P4 is a pixel matrixP on the side close to the driving scanning section 105, the pixelcircuit P5 is a pixel matrix P at the middle of the pixel row and thepixel circuit P6 is a pixel circuit P on the side close to the rightedge of the display panel section 100.

In the typical configuration shown in the block diagram of FIG. 10,reference notation r denotes the wire resistance exhibited by thepower-supply providing line 105DSL as a wire resistance per length unitcorresponding to one pixel circuit P. On the other hand, referencenotation R denotes the wire resistance of the draw wire 121DL connectingthe drain electrode of the driving transistor 121 employed in everypixel circuit P to the power-supply providing line 105DSL. Moreparticularly, each of reference notations R1, R2, R3, R4, R5 and R6shown in the block diagram of FIG. 10 denotes the wire resistance of thedraw wire 121DL connecting the drain electrode of the driving transistor121 employed in one of the pixel circuits P1, P2, P3, P4, P5 and P6respectively. A suffix is attached to reference notation R to serve as asuffix matching a suffix attached to reference notation P denoting apixel circuit including the draw wire 121DL with a wire resistancedenoted by reference notation R.

In an operation to show an all white display on the screen, basically, adriving current Ids (=I) uniform for all pixel circuits P flows througheach driving transistor 121. Since the driving current I is generated bythe driving scanning section 105 and flows to the driving transistor 121in any pixel circuit P through the power-supply providing line 105DSLand the draw wire 121DL provided for the driving transistor 121, anelectric potential appearing on the drain electrode of the drivingtransistor 121 decreases by a voltage drop caused by the resistances ofthe power-supply providing line 105DSL and the draw wire 121DL. Let eachof reference notations V1, V2, V3, V4, V5 and V6 shown in the blockdiagram of FIG. 10 denote the electric potential appearing on the drainelectrode of the driving transistor 121 employed in one of the pixelcircuits P1, P2, P3, P4, P5 and P6 respectively. A suffix is attached toreference notation V to serve as a suffix matching a suffix attached toreference notation P denoting a pixel circuit including the drivingtransistor 121 having the drain electrode on which the electricpotential denoted by reference notation V appears.

Let each of reference notations ΔV1, ΔV2, ΔV3, ΔV41, ΔV5 and ΔV6 denoterespectively a voltage drop from a voltage appearing at the outputterminal of the driving scanning section 105 to one of the electricpotentials V1, V2, V3, V4, V5 and V6 appearing on the drain electrode ofa driving transistor 121. In this case, the voltage drops ΔV1, ΔV2, ΔV3,ΔV4, ΔV5 and ΔV6 can be expressed by Eqs. (3) as follows:ΔV1=r×3I+R1×IΔV2=r×3I+r×2I+R2×IΔV3=r×3I+r×2I+r×I+R3×IΔV4=r×3I+R4×IΔV5=r×3I+r×2I+R5×IΔV6=r×3I+r×2I+r×I+R6×I  (3)

Even if the voltage appearing at the output terminal of the drivingscanning section 105 drops to the electric potentials V1, V2, V3, V4, V5and V6 appearing on the drain electrodes of the driving transistors 121,no display irregularities are generated on the screen provided that thevoltage drops are uniform for all the driving transistors 121. Themechanism according to the embodiment is designed by paying attention tothe fact that no display irregularities are generated on the screenprovided that the voltage drops from the voltage appearing at the outputterminal of the driving scanning section 105 to the electric potentialsV1, V2, V3, V4, V5 and V6 appearing on the drain electrodes of thedriving transistors 121 are uniform for all the driving transistors 121.If the voltage drops from the voltage appearing at the output terminalof the driving scanning section 105 to the electric potentials V1, V2,V3, V4, V5 and V6 appearing on the drain electrodes of the drivingtransistors 121 are uniform for all the driving transistors 121, Eqs.(4) can be derived from Eqs. (3) as follows:

$\begin{matrix}\begin{matrix}{{{r \times 3I} + {R\; 1 \times I}} = {{r \times 3I} + {r \times 2I} + {R\; 2 \times I}}} \\{= {{r \times 3I} + {r \times 2I} + {r \times I} + {R\; 3 \times I}}} \\{= {{r \times 3I} + {R\; 4 \times I}}} \\{= {{r \times 3I} + {r \times 2I} + {R\; 5 \times I}}} \\{= {{r \times 3I} + {r \times 2I} + {r \times I} + {R\; 6 \times I}}}\end{matrix} & (4)\end{matrix}$

The wire resistance R of the draw wire 121DL also referred to as a drainwire provided for each pixel circuit P is adjusted so that Eqs. (4) aresatisfied. That is to say, the values of the wire resistances R1, R2,R3, R4, R5 and R6 are found by solving Eqs. (4). The wire resistancesR1, R2, R3, R4, R5 and R6 satisfying Eqs. (4) provide an electricpotential V appearing on the drain electrode of the driving transistor121 employed in each pixel circuit P as an electric potential V uniformfor all the pixel circuits P as expressed by the following equations:V1=V2=V3=V4=V5=V6

The wire resistance R of the draw wire 121DL is adjusted for each of thepixel circuits P arranged in the longitudinal direction of thepower-supply providing line 105DSL by properly changing the lengthand/or width of the draw wire 121DL.

To put it concretely, the wire resistance R of the draw wire 121DLconnecting the drain electrode of the driving transistor 121 employed ina pixel circuit P close to the driving scanning section 105 to thepower-supply providing line 105DSL is increased by reducing the width ofthe draw wire 121DL and/or increasing the length of the draw wire 121DL.On the other hand, the wire resistance R of the draw wire 121DLconnecting the drain electrode of the driving transistor 121 employed ina pixel circuit P far away from the driving scanning section 105 to thepower-supply providing line 105DSL is decreased by increasing the widthof the draw wire 121DL and/or reducing the length of the draw wire121DL. Such draw wires 121DL are laid out in the pixel array section 102so as to provide an electric potential V appearing on the drainelectrode of the driving transistor 121 employed in each pixel circuit Pas an electric potential V uniform for all the pixel circuits P.

By adjusting the length and/or width of the draw wire 121DL provided inevery pixel circuit P and laying out such draw wires 121DL in the pixelarray section 102, it is possible to provide an electric potential Vappearing on the drain electrode of the driving transistor 121 employedin each pixel circuit P as an electric potential V uniform for all thepixel circuits P. As explained before, the drain electrode of a drivingtransistor 121 serves as the power-supply providing terminal of thedriving transistor 121. Even if the voltage appearing at the outputterminal of the driving scanning section 105 drops to the electricpotentials V1, V2, V3, V4, V5 and V6 appearing on the drain electrodesof the driving transistors 121 due to the driving current Ids flowingthrough the power-supply providing line 105DSL and draw wires 121DL, nodisplay irregularities are generated on the screen provided that thevoltage drops are uniform for all the driving transistors 121.

<First Embodiment>

Each of FIGS. 11A, 11B, 12, 13A and 13B is an explanatory diagramserving as a model diagram referred to in description of a typicallayout according to a first embodiment implementing the basic principleexplained earlier by referring to the diagram of FIG. 10. To be morespecific, FIGS. 11A and 11B are diagrams each serving as a model diagramreferred to in description of a typical layout according to none offirst to third embodiments to be described later. On the other hand,FIG. 12 is a diagram serving as a model diagram referred to indescription of a typical layout according to the first embodiment. It isto be noted that, since a relation between the power-supply providingline 105DSL and the draw wire 121DL is a relation of most importance tothe mechanism according to the embodiments, other portions are displayedin a simple manner by showing a pixel circuit P. The completeconfiguration of the pixel circuit P is also shown in a circuit diagramof FIG. 11A for the sake of clarity.

At any rate, in order to reduce the resistances of the write scanningline 104WS, the power-supply providing line 105DSL and the video-signalline 106HS, each of the write scanning line 104WS, the power-supplyproviding line 105DSL and the video-signal line 106HS is made from alow-resistivity material such as aluminum or tungsten and implemented onan second wiring layer L2. It is to be noted that a first wiring layerL1 also provided for the video-signal line 106HS made of a differentmaterial such as molybdenum having a relatively larger resistanceoverlaps an intersection of the video-signal line 106HS and the writescanning line 104WS as well as an intersection of the video-signal line106HS and the power-supply providing line 105DSL. In the typical layoutshown in the diagram of FIG. 11B, at each of an intersection of thevideo-signal line 106HS and the write scanning line 104WS and anintersection of the video-signal line 106HS and the power-supplyproviding line 105DSL, a portion of the video-signal line 106HS is madeto pass through the first wiring layer L1 provided for the intersection.

A draw line connecting a terminal of a transistor to a scanning line istypically implemented on the first wiring layer L1 or the second wiringlayer L2 or implemented on a layer other than the first wiring layer L1or the second wiring layer L2. For example, in the typical comparisonlayout shown in the diagram of FIG. 11B, a gate draw wire 125GL madefrom the same material as the video-signal line 106HS implemented on thefirst wiring layer L1 is used for connecting the gate electrode of thesampling transistor 125 to the write scanning line 104WS. The gate drawwire 125GL is also referred to merely as a gate line. On the other hand,the draw wire 121DL implemented on a third wiring layer L3 is used forconnecting the drain electrode of the driving transistor 121 to thepower-supply providing line 105DSL. The draw wire 121DL implemented onthird wiring layer L3 is made of a material different from the materialsused for making the write scanning line 104WS, the power-supplyproviding line 105DSL and the video-signal line 106HS on the firstwiring layer L1 and the second wiring layer L2. The material used formaking the draw wire 121DL on the third wiring layer L3 has a largeresistivity in comparison with the material used for making the writescanning line 104WS, the power-supply providing line 105DSL and thevideo-signal line 106HS on the second wiring layer L2. That is to say,for the same width, the same length and the same thickness, the drawwire 121DL implemented on the third wiring layer L3 has a highresistance in comparison with the second wiring layer L2. A poly-siliconlayer is a typical example of the material for making the draw wire121DL implemented on the third wiring layer L3.

In the typical comparison layout shown in the diagram of FIG. 11B, thedraw wire 121DL is connected to the power-supply providing line 105DSLin a simple manner. The wire shape of the draw wire 121DL is uniform forall pixel circuits P. That is to say, the draw wire 121DL has a layoutuniform for all pixel circuits P.

In a layout according to the first embodiment shown in the diagram ofFIG. 12, on the other hand, the resistance of the draw wire 121DL oneach pixel circuit P close to the driving scanning section 105 is maderelatively large whereas the resistance of the draw wire 121DL on eachpixel circuit P far away from the driving scanning section 105 is maderelatively small. As shown in the diagram of FIG. 10, pixel circuits Pclose to the driving scanning section 105 are pixel circuits P1 and P4whereas pixel circuits P far away from the driving scanning section 105are pixel circuits P3 and P6.

That is to say, the length of the draw wire 121DL on any pixel circuit Pis set at such a value that, the longer the distance between the drivingscanning section 105 and the pixel circuit P, the smaller the resistanceof the draw wire 121DL. To put it concretely, in the layout according tothe first embodiment shown in the diagram of FIG. 12, the draw wire121DL includes a first draw wire 121DL_1 connected to the drainelectrode of the driving transistor 121 and a second draw wire 121DL_2connected to the power-supply providing line 105DSL. The first draw wire121DL_1 on the drain side is made all but identical with that of thetypical comparison layout shown in the diagram of FIG. 11B. However, thefirst draw wire 121DL_1 is extended by the second draw wire 121DL_2 usedas an extension wire to contacts on the power-supply providing line105DSL and the length of the second draw wire 121DL_2 is adjusted inaccordance with the location of a pixel circuit P including the firstdraw wire 121DL_1. In this way, the length of the entire draw wire 121DLon any pixel circuit P close to the driving scanning section 105 orclose to the input edge side is made relatively large whereas the lengthof the entire draw wire 121DL on any pixel circuit P far away from thedriving scanning section 105 or far away from the input edge side ismade relatively small. That is to say, the length of the entire drawwire 121DL on any pixel circuit P is adjusted so that, the longer thedistance between the driving scanning section 105 and the pixel circuitP, the smaller the length of the entire draw wire 121DL. By the way, onthe pixel circuit P farthest from the driving scanning section 105, thatis, the pixel circuit P on the output edge side of the right edge sideof the display panel section 100, the second draw wire 121DL_2 is notused as an extension line in order to make the length of the entire drawwire 121DL smallest.

The draw wire 121DL including the first draw wire 121DL_1 and the seconddraw wire 121DL_2 is implemented on the third wiring layer L3 parallelto the second wiring layer L2 implementing the power-supply providingline 105DSL separately from the power-supply providing line 105DSL. Asdescribed before, draw wire 121DL implemented on third wiring layer L3is made of a material such as poly-silicon. By implementing the drawwire 121DL in this way, the draw wire 121DL by no means serves as abarrier to the layouts of other wires. Then, after finding theresistance R of the draw wire 121DL in accordance with Eqs. (4) in orderto determine the length of the second draw wire 121DL_2 for every pixelcircuit P, every second draw wire 121DL_2 is connected to the contactscited before. In the case of a pixel circuit P having no second drawwire 121DL_2, the first draw wire 121DL_1 is connected directly to thecontacts.

In addition, the draw wire 121DL is implemented on the third wiringlayer L3 and made of a wire material having a large resistivity incomparison with the material for making the second wiring layer L2 whichimplements the power-supply providing line 105DSL. Thus, the process offinding the resistance R of the draw wire 121DL in accordance with Eqs.(4) in order to determine the length of the second draw wire 121DL_2 forevery pixel circuit P is easy to carry out.

<Second Embodiment>

FIG. 13A is an explanatory diagram serving as a model diagram referredto in description of a typical layout according to a second embodimentimplementing the basic principle explained earlier by referring to thediagram of FIG. 10. In an implemented mechanism shown in the diagram ofFIG. 10, the resistance of the draw wire 121DL on each pixel circuit Pclose to the driving scanning section 105 is made relatively largewhereas the resistance of the draw wire 121DL on each pixel circuit Pfar away from the driving scanning section 105 is made relatively small.As shown in the diagram of FIG. 10, pixel circuits P close to thedriving scanning section 105 are pixel circuits P1 and P4 whereas pixelcircuits P far away from the driving scanning section 105 are pixelcircuits P3 and P6.

That is to say, the width of the draw wire 121DL on any pixel circuit Pis determined for every pixel circuit P so that, the longer the distancefrom the driving scanning section 105 to a pixel circuit P, the largerthe value at which the width of the draw wire 121DL provided for thepixel circuit P is set. To put it concretely, in a layout according tothe second embodiment, instead of adjusting the length of the seconddraw wire 121DL_2, the width of the draw wire 121DL on each pixelcircuit P close to the driving scanning section 105 is set at arelatively small value whereas the width of the draw wire 121DL on eachpixel circuit P far away from the driving scanning section 105 is set ata relatively large value.

The draw wire 121DL is implemented on the third wiring layer L3 parallelto the second wiring layer L2 implementing the power-supply providingline 105DSL separately from the power-supply providing line 105DSL. Asdescribed before, draw wire 121DL implemented on third wiring layer L3is made of a material such as poly-silicon. By implementing the drawwire 121DL in this way, the draw wire 121DL by no means serves as abarrier to the layouts of other wires.

By the way, on a pixel circuit P far away from the driving scanningsection 105, the number of contacts connected to the draw wire 121DL canbe raised to a value proper for the increased width of the draw wire121DL. As an alternative, the number of contacts connected to the drawwire 121DL can be made uniform for all pixel circuits P.

<Third Embodiment>

FIG. 13B is an explanatory diagram serving as a model diagram referredto in description of a typical layout according to a third embodimentimplementing the basic principle explained earlier by referring to thediagram of FIG. 10. In an implemented mechanism shown in the diagram ofFIG. 10, the resistance of the draw wire 121DL on each pixel circuit Pclose to the driving scanning section 105 is made relatively largewhereas the resistance of the draw wire 121DL on each pixel circuit Pfar away from the driving scanning section 105 is made relatively small.As shown in the diagram of FIG. 10, pixel circuits P close to thedriving scanning section 105 are pixel circuits P1 and P4 whereas pixelcircuits P far away from the driving scanning section 105 are pixelcircuits P3 and P6.

That is to say, the length and width of the draw wire 121DL on any pixelcircuit P are determined for every pixel circuit P so that, the longerthe distance from the driving scanning section 105 to a pixel circuit P,the smaller the value at which the length of the draw wire 121DLprovided for the pixel circuit P is set and the larger the value atwhich the width of the draw wire 121DL provided for the pixel circuit Pis set. To put it concretely, in a layout according to the thirdembodiment obtained by combining the first and second embodiments, thelength of the draw wire 121DL on each pixel circuit P close to thedriving scanning section 105 is set at a relatively large value whereasthe width of the draw wire 121DL on each pixel circuit P close to thedriving scanning section 105 is set at a relatively small value. On theother hand, the length of the draw wire 121DL on each pixel circuit Pfar away from the driving scanning section 105 is set at a relativelysmall value whereas the width of the draw wire 121DL on each pixelcircuit P far away from the driving scanning section 105 is set at arelatively large value.

In the same way as the first embodiment, the draw wire 121DL includingthe first draw wire 121DL_1 and the second draw wire 121DL_2 isimplemented on the third wiring layer L3 parallel to the second wiringlayer L2 implementing the power-supply providing line 105DSL separatelyfrom the power-supply providing line 105DSL. As described before, thedraw wire 121DL implemented on third wiring layer L3 is made of amaterial such as poly-silicon. By implementing the draw wire 121DL inthis way, each of the first draw wire 121DL_1 and the second draw wire121DL_2 by no means serves as a barrier to the layouts of other wires.

By adjusting both the length and width of the second draw wire 121DL_2in accordance with the third embodiment as described above, the range ofthe resistance of the draw wire 121DL becomes wider than the ranges eachprovided by the first or second embodiment as the range of theresistance of the draw wire 121DL.

By designing a pixel layout in accordance with any one of the first tothird embodiments as described above, it is possible to repress theshading phenomenon shown in the diagram of FIG. 8 and the crosstalkphenomenon shown in the diagram of FIG. 9C. The shading and crosstalkphenomena are caused by voltage drops at a plurality of points along thepower-supply providing line 105DSL, becoming a problem raised by thetypical comparison pixel layout shown in the diagrams of FIGS. 11A and11B. Thus, in a display apparatus employing pixel circuits each having acurrent-driven-emission light emitting device such as the organic ELdevice 127, the shading and crosstalk phenomena can be repressed. As aresult, a good image quality can be obtained.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2008-106456 filedin the Japan Patent Office on Apr. 16, 2008, the entire content of whichis hereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factor in so far as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A display apparatus comprising: a pixel arraysection comprising: a plurality of pixel circuits arranged in a matrixand including a first pixel circuit and a second pixel circuit, thefirst pixel circuit including a first driving transistor and a firstelectro-optical device, the second pixel circuit including a seconddriving transistor and a second electro-optical device; and draw wiresincluding a first draw wire for the first pixel circuit and a seconddraw wire for the second pixel circuit, wherein the first draw wire isprovided in said first pixel circuit to connect said first drivingtransistor to a power supply line via a first contact area, the seconddraw wire is provided in said second pixel circuit to connect saidsecond driving transistor to said power supply line via a second contactarea, the first pixel circuit is located closer to a source applying apower-supply voltage to said power supply line than the second pixelcircuit, and a size of the first contact area is larger than a size ofthe second contact area.
 2. The display apparatus according to claim 1wherein the resistance of the first draw wire is larger than theresistance of the second draw wire because of a difference between atleast one of the length or the width of the first draw wire as comparedto at least one of the length or width of the second draw wire.
 3. Thedisplay apparatus according to claim 2 wherein at least one of thelength or width of said first and second draw wires are respectivelyvaried for respective pixel circuits so that an electric potentialappearing on power-supply providing terminals of said first and seconddriving transistors becomes uniform for said first and second pixelcircuits under a condition of driving said first and secondelectro-optical devices.
 4. The display apparatus according to claim 2wherein: said first and second draw wires are created on a draw-wirewiring layer separated away from a power-supply wiring layer of saidpower-supply line; and a wire portion included in respective ones ofsaid draw wires, the length or the width of which is varied inaccordance with the location of said pixel circuit including said drawline, is implemented on said draw-wire wiring layer parallel to saidpower-supply wiring layer of said power supply line.
 5. The displayapparatus according to claim 1 wherein a wire material used for makingsaid draw wires has a resistivity greater than the resistivity of a wirematerial used for making said power supply line.
 6. The displayapparatus according to claim 1, further comprising a driving-signalstabilization circuit for stabilizing a driving current generated bysaid driving transistor.
 7. The display apparatus according to claim 6wherein at least said first and second pixel circuits respectively havefirst and second sampling transistors for respectively passing a videosignal to first and second signal holding capacitors corresponding tosaid first and second driving transistors with a timing determined inadvance, and for at least said first pixel circuit, said driving-signalstabilization circuit is configured to: supply said video signal to saidfirst sampling transistor by switching the electric potential of saidvideo signal from a reference electric potential to a signal electricpotential and vice versa; and carry out a threshold-voltage compensationfunction to hold a voltage corresponding to the threshold voltage ofsaid first driving transistor in said first signal holding capacitor byputting said first sampling transistor in a turned-on state during atime band in which a voltage corresponding to a first electric potentialused for causing a driving current to flow to said first electro-opticaldevice is being applied to a power-supply providing terminal of saidfirst driving transistor whereas said reference electric potential ofsaid video signal is being applied to said first sampling transistor. 8.The display apparatus according to claim 6 wherein at least said firstand second pixel circuits respectively have first and second samplingtransistors for respectively passing a video signal to first and secondsignal holding capacitators corresponding to said first and seconddriving transistors, and for at least said first pixel circuit, saiddriving-signal stabilization circuit is configured to carry out: athreshold-voltage compensation function to hold a voltage correspondingto the threshold voltage of said first driving transistor in said firstsignal holding capacitor; and a mobility compensation function to add acompensation voltage corresponding to the mobility of said first drivingtransistor to a signal to be stored in said first signal holdingcapacitor in an operation to store said signal amplitude into said firstsignal holding capacitor as video information by putting said firstsampling transistor in a turned-on state after completion of executionof said threshold-voltage compensation function.
 9. The displayapparatus according to claim 6 wherein at least said first and secondpixel circuits respectively have first and second signal holdingcapacitors, and for at least said first pixel circuit, saiddriving-signal stabilization circuit is configured to carry out abootstrap function based on a coupling effect of said first signalholding capacitor which is connected between a control input terminal ofsaid first driving transistor and a driving-current output terminal ofsaid first driving transistor.